Learn English – What were—or are—”odd pants?”

In the journey from Old English to what we write today, the ash (Æ) tended to metamorphose into a simple E and various "ae" forms got reduced to just "e": Ælfwyn became Elvin, Æthelræd became Ethelred, aether and aesthetic became ether and esthetic (except when @Cerb spells them), and so on. The distinction was simply planed off over the centuries. When there was no need for the superfluous Æ (because its sound was rendered with a single letter) it got dialed way down in frequency. The same thing happened, more or less, with the thorn (Þ, þ) and eth (Ð, ð) characters, because the th digraph supplanted them.

An even more obvious influence involved the printing press. In the early days of typography, fonts were imported from Germany and Italy, and those countries did not use the oddball English characters, so substitutes had to be found. "E" substituted quite nicely for the ash, and "Y" for the thorn (as we see on the signs in front of all those cutesy Ye Old Whatever shops).

Elaboration

Asked for citations, I lazily looked to the Web first, but real scholarship in this matter is difficult to Google. Here are some not-stringently-academic citations, together with a disclaimer.

The thorn was particularly popular as a sign for 'th' in Medieval English, but with the advent of printing came a problem. There was no thorn sign in the printing fonts, as they were usually cast outside of England. So, since the sign for thorn slightly resembled the lower-case 'y', that's what was substituted.

The thorn continued to be used, but printing caused its eventual demise from the English alphabet. As mentioned earlier, lingering proof of its existence hangs on in the outmoded 'Ye'.
Thorn — Missing Letter of the Alphabet (reposted with correct glyphs here).

Ultimately, the letter was abandoned when printing began to streamline the alphabet and eliminate unnecessary letters. Æ was separated into AE, and the language moved on. However, you can still find ash used stylistically in names like Encyclopædia Britannica and ÆON.
Mighty Markup.

Disclaimer: I feel it only fair to point out that the reference books I have at hand (printed versions, so no linky-link), suggest that the ash (or æsc in OE), was pretty much gone by 1250 due to the influence of Norman French. This was a couple hundred years before the invention of the printing press, so we cannot accept that as the proximal cause. Still, Gutenberg almost certainly put the nail in the coffin of that and the other oddball characters (including wynn and yogh — look those up for your amusement and edification sometime).

The origin of the term frog in relation to the braid on military uniforms (and almost uniquely cavalry uniforms) might come from the old term for frock. The OED has:

Frock 2. An upper garment worn chiefly by men... 1375. Barbour Bruce x. 375 With blak froggis all helit thai The Armouris at thai on thame had. c 1425 Wyntoun Cron. viii. xxxviii. 57 Ilkane a gud Burdowne in hand, And royd Frogis on þare Armyng. c 1460 Towneley Myst. (Surtees) 241, I wold be fayn of this frog [Christ's coat] myght it fall vnto me. 1500–20 Dunbar Poems li. 3 To giff a doublett he is als doure, As it war off ane futt syd frog

A 14th Century 'frock' or 'frog'. Note the belt and cord carrying gear. The term frog-loop and frog-belt have been used to describe these items up to the 19th Century (see below) and even today a frog-loop is still a term for a retainer for a tool in a tradesman's belt.

There are also references in the Dictionary of the Scots Language:

Frog, Froig, n.1 [ME. frog, frogge (15th c.), of uncertain origin.] A frock; a cloak or coat. Barb. x. 375. With blak froggis all helyt thai The armouris at thai on thame had; Wynt. viii. 5702. [They had] royd frogis on thare armyng, To covyre thame for persaywyng; Crying of Play 39. Five thousand ellis ȝeid in his frog Of hieland pladdis of haire; Dunb. li. 3. To giff a doublett he is als doure, As it war off ane futt syd frog; Doug. vi. v. 132. In hevy wayt frog stad and chargyt soyr; 1550–1 Treas. Acc. IX. 467. Spanȝe freis … to be ane froig to my Lady;

So what we might be looking at is frog(frock)-button, and frog(frock)-belt, as below:

1719 De Foe Crusoe ii. iv. (1840) II. 68 He drew a hatchet out of a frog-belt. 1827 Hone Every-day Bk. II. 190 A coat with frog-buttons. 1867 Smyth Sailor's Word-bk., Frog-belt, a baldrick.

Curiously (and perhaps destructively to this argument) Defoe 1719 also has:

1719 De Foe Crusoe i. xv, A belt with a frog hanging to it, such as..we wear hangers in.

Tailoring of simple jackets, frocks or 'frogs' only became usual from the 1500's onwards. Prior to that they were held in shape with belts, cords, lacing and over the shoulder straps which served the dual function of carrying tools and possessions on hooks and loops, and in bags hung from them. Pockets are a relatively late invention in clothing. (See: http://www.theinnerbailey.com/baileybasics-sets-townsman-1.jpg)

My suggestion (tending towards an answer) is that this early fashion of belting, lacing and buttoning was contemporary with the use of the ME word 'frog' for jacket, and the the technical expressions for items associated with them such as frog-button, frog-belt, frog-loop and frogging were all developed during that time. As fashion evolved in the 1500-1600 period and clothes were more tailored to the shape of the body and pockets came into use, not only did the term 'frog' evolve into 'frock', but those belts and cords became less important for shaping the material and for carrying items. Frog as a word for a frock evolved, but the use of frog in association with those accoutrements became fossilized.

To see how these fossilized terms for buttoning and braiding (frog-buttons, frogging) came to be associated with a military fashion of the early 1700's (in England) we need to look at Eastern Europe in the late 1400's.

Frogging (as we call it) is most closely, and most extravagantly, associated with lightly armoured mounted troops known as Hussars or Dragoons. This link to an image from c.1550 clearly shows the horizontal braid on a mounted Hungarian soldier's uniform:

Such troops, and their dress styles, evolved in Eastern Europe before spreading to Western Europe in the late 1600's. None of the languages or cultures (including that of the Ottoman Empire) appear to have contributed the word 'frog' or 'frogging' along the way.

The actual fashion which became quickly extravagant and an actual impediment to military action (1746 Berkeley Let. Wks. 1871 IV. 306 Laces, frogs, cockades..are so many..obstacles to a soldier's exerting his strength.) may have originated in loops in the jacket to hold additional weapons such as daggers and short swords, or in the lacing used to tighten loose plates of armour or a baggy cloth jacket around the body.

When the new style reached England in the mid 1700's there was no current word to describe the braiding (that had become increasingly ornate as it travelled west across Europe). It was recognized, however, as similar to the ornamentation formerly (and no longer) used on the item of clothing once known as a frog. The names for that ornamentation (including 'frogging') had never evolved because the items had been largely disused for about a hundred years, but were revived in their original form to describe the ornamentation on this new (to English eyes) military fashion.

This speculative essay into the evolution of the use of the word 'frog', and the suggestion that it's original association with the article of clothing, and the accoutrements to that clothing went down different paths, and that the word for the accoutrements was frozen for some one or two hundred years while the word for the clothing evolved, and how the frozen word was revived to describe a new fashion element, also explains how we arrived at DeFoes (otherwise improbable) description of a 'frock with frog-buttons'. Essentially he is saying, 'a frock with frock-buttons', but the language available to him has him say otherwise.