Learn English – Where does the word “button-down” come from

You are correct about the current usage. You would not use it to refer to a mental patient in the technical sense, but apparently it was at one time acceptable to do so. I would have thought that the idiom started life in the "angry and agitated" sense, but there is one old (1804) citation that I found via an ngrams search which uses the second sense, apparently without irony. I do not know when the newer sense of the phrase took over, but it was certainly in use about 100 years after that first example.

"Mad" itself seems to have followed the same path. The earliest citations in the OED refer to the "mental disease" aspect, and the "uncontrollable rage" meaning comes later, followed by the colloquial sense of "angry".

It seems to be a common pattern, that a technical phrase for a mental condition gets borrowed for colloquial use and then falls out of usage in its original sense to avoid offense. "Retarded" is another example: the use of the word as an insult and "synonym" for "unintelligent" put it onto the Euphemism treadmill.

One thing you can do with terms with unknown origin is to consider all possible sources. Here is a list of the more common theories about humbug etymology;

Humbug, from (maps.thefullwiki.org):

• It has also existed in many other countries, unconnected with the British Empire, for a long time. For instance, in Germany it has been known since the 1830s, in Sweden since at least 1862, in French since at least 1875, in Hungarian,, Russian and in Finnish

• The oldest known written uses of the word are in the book The Student (1750-1751), ii. 41, where it is called "a word very much in vogue with the people of taste and fashion." and in Ferdinando Killigrew's The Universal Jester, subtitled "a choice collection of many conceits ... bonmots and humbugs" from 1754; as mentioned in Encyclopædia Britannica from 1911, which further refers to the New English Dictionary.

There are many theories as to the origin of the term, none of which have been proven:

• Charles Godfrey Leland mentions the idea that the word could be derived from the Norse word hum, meaning 'night' or 'shadow', and the word bugges (used in the Bible), a variant of bogey, meaning 'apparitions'. The Norse word hum mentioned, or hume, actually means 'dark air' in Old Norwegian. From the other Scandinavian languages based on Old Norse, we have hüm in Icelandic which means 'twilight', hómi in Faeroese which means 'unclear', and humi in Old Swedish which means 'dark suspicion', documented back to 1541. From this word is also derived the Swedish verb hymla, still in use, which means 'to conceal, hide, not commit to the truth'.

• According to the Dictionary of the Vulgar Tongue by Francis Grose, 1731-1791, to hum in English indeed originally meant 'to deceive'. To combine this early medieval Scandinavian word with bugges from the English Bible of a later date may seem far-fetched. But it is however plausible that it could have been combined with the much older Celtic word bwg, meaning 'ghost', due to the Viking conquests of the British Isles at the time, which have much influenced English. Bwg is also what developed into "bugges" in Middle English 1350-1400, then with the meaning of scarecrow or similar. This older connection makes more sense since apparently the term's origin was already unknown in 1751. Also, with bwg meaning ghost, the use of the term fits nicely in Dickens's novel about the Christmas ghosts. In Etym. Diet. of 1898, Walter Skeat also proposed a similar theory, although using contemporary versions of the words, where hum meant to murmur applause, and bug being a spectre.

• It could also come from the Italian uomo bugiardo, which literally means 'lying man'. There was considerable Italian influence on English at the time (e.g. Shakespeare's numerous Italian-based plays).

• Uim-bog is supposed to mean 'soft copper' in Irish, worthless money, but there is no evidence of a clear connection to the term.

• The 1911 Encyclopædia Britannica also suggests that it is a form of "Hamburg", where false coins were minted and shipped to England during the Napoleonic wars, which is inaccurate as the Napoleonic wars were 50 years after the word first appeared in print.

• A modern conception is that it actually refers to a humming bug—i.e. something small and inconsequential, such as a cicada, that makes a lot of noise.

References: (www.americanheritage.com) www.etymologie.info Svenska Akademiens ordbok s.v. Humbug Dictionnaire de la Langue Francais, Littré 1872-1877 Wiktionary V.K. Müller, Russian-English Dictionary 1911encyclopedia.org Charles Godfrey Leland, Gypsy Sorcery and Fortune Telling [1891], Chapter X of The Haunts, Homes, and Habits of Witches in The South Slavic Lands--Bogeys and Humbugs Svenska Akademiens ordbok s.v. Hum Svenska Akademiens ordbok s.v. Hymla Dictionary of the Vulgar Tongue dictionary.com 1911 Encyclopedia s.v. Humbug Factmonster 1911 Encyclopedia 1911 Encyclopedia word-detective