The OED says this verb chicken is slang of US origin with a first quotation from 1943 (I. Wolfert, Torpedo 8):
I just wanted to..make sure you weren't chickening out on me.
They say this is a revived form coming from a noun chicken for one who is as timorous or defenceless as a chicken, used at least as early as 1616, and cite Shakespeare (Cymbeline 1623):
Forthwith they flye Chickens, the way which they stopt Eagles.
Hawthorne
The adjective peachy keen was popularised and probably invented by LA DJ Jim Hawthorne around 1948.
Time Magazine
The OED has peachy-keen from 1951, but here's a couple of antedatings from Time Magazine in 1948.
First from Monday, May 10, 1948:
Radio: Peachy-Keen
Jim Hawthorne, a young Pasadena disc jockey, used to be bored with his job ($85 a week). Sometimes he would sign off with a sneer: "This is KXLA, the 10,000-watt jukebox." But he is bored with his job no longer.
One night, without notifying his bosses, Hawthorne suddenly turned his show into a carefree, wit-loose "Hellzapoppin on the air." Next day, before the station had time to fire him, the place was snowed under with fan mail. By last week, the scattyboo platter session was being broadcast over five Southern California stations ("the net-to-net coastwork of the Oh-So-Peachy-Keen Broadcasting Company").Both ABC and Mutual were dickering for national network rights. Hawthorne's salary is now $450 a week. The Hawthorne formula is a well- stirred ragout of one part Henry Morgan, three parts Arthur Godfrey and a dash of Colonel Stoopnagle; it is a blend of the outrageously unexpected and the shaggy dog joke. In the middle of a recording, voice may suddenly announce: "I've got cole slaw in all my pockets I'm cold." Sometimes Hawthorne heckles his lovesick records. "What are you in the mood for, honey?" he will ask during the opening bars of a song. "I'm in the mood for love," the record croons back.
Whatever adults — and sponsors — may think of such carryings-on, Hawthorne and his peculiar banana-split lingo have become the rage of Southern California's younger set. Most popular root word is "hogan" (example: "I was driving my carahogan in from Pasadena-hogan so I could get a hoganburger"). The young folks also overwork Hawthorne's favorite adjectives: keen, peachy-keen, and oh-so- peachy-keen.
Next from Monday, Oct. 11, 1948:
Music: Gumbo
The piece begins with a wolf call, and ends with all the instruments thrown into a corner. It is scored for ukulele, kazoo, hogan-twanger (wooden box and hacksaw blades), cardboard box, seal barks and an Indian elephant bell. It has words like this:
The boy I mean was oh-so peachy-keen,
A real gone guy from Goneville.
He was scattyboo and oogledy-too,
And he lived in Pasahogan.
The title of the piece spells Nature's Boy backwards. By last week Serutan Yob had sold 350,000 records, and Capitol was threatening to make at least a million.
(Both use the word scattyboo and other contemporary slang.)
Listen here and read the lyrics in full here. Wikipedia says:
A parody named "Serutan Yob" was recorded by The Unnatural Seven, an offshoot of Red Ingle and his Natural Seven that did not include Ingle due to the 1948 AFM recording ban. The record featured vocals from Karen Tedder and Los Angeles DJ Jim Hawthorne. It was released by Capitol Records as catalog number 15210. The record first reached the Billboard magazine charts on 1 October 1948 and lasted 4 weeks on the chart, peaking at No. 24.
There's that DJ's name again, Jim Hawthorne, who delivered the peachy-keen lines quoted above. The book Echo and Reverb: Fabricating Space in Popular Music Recording, 1900-1960 says it was released in August 1948 and can be found in the 11 September 1948 Billboard Magazine.
Best Answer
Merriam-Webster's Eleventh Collegiate Dictionary (2003) gives a straightforward summary of the etymology of trinity:
What this summary tells you is that the word Trinity appeared in English from Middle English in the thirteenth century; that it was derived ultimately from the Latin word for "threefold," and that its original meaning in English was the religious one indicating the three-in-oneness of Jesus/God/Holy Ghost.
The earliest instance that a Google Books search for "Holy Trinity" turns up is from Articles agreed upon by the Bishops, and other Learned and Godly Men, in the last Convocation at London, in the year of our Lord 1552. To root out the Discord of Opinions, and establish the Agreement of TRUE RELIGION. Published By the Kings Majesties Authority, 1553. (1553):
The king was Edward VI, in what proved to be the final year of his reign.
Update: A look at early matches for 'Trinite' and an earliest match for 'holy Trinite'
In hopes of finding an earlier instance of "Holy Trinity," I ran a Google Books search for Trinite, the predecessor word in Middle English to Trinity. The most interesting result was a concordance to the A, B, and C versions of William Langland, William's Vision of Piers Ploughman, the famous medieval allegorical poem, which Wikipedia reports was written circa 1370–1390.
The concordance turns up several dozen matches for Trinite [or trinite], and it finds even more matches for holy (particularly in the combinations holy chirche [or churche or kyrke or kirke], holy gost [or goste or goost], and holy writ, but also occasionally in the forms holy day [or dayes or daies], holy men, and (once each) holy Seintes and holy euen. But there is only is only one instance in which holy and Trinite appear in the same line, and in that case holy modifies goost:
Virtually all instances of Trinite in Piers Ploughman are preceded by þe. This proves nothing about whether some contemporaries of William Langland used the term holy Trinite; but it does provide fairly strong evidence that in the late 1300s holy Trinite was not a set phrase in Langland's part of England the way that holy chirche, holy gost, and holy writ were.
In Tarjei Park, "Reflecting Christ: The Role of the Flesh in Walter Hilton and Julian of Norwich," in The Medieval Mystical Tradition in England: Exeter Symposium V (1992), the academic author reproduces several quotations from Julian of Norwich's Revelations of Divine Love (ca. 1395), including one that comes close to "holy Trinity":
Most of Julian's mentions of Trinite are unmodified, but the last one, blissid Trinite, comes close to holy Trinite. Still her choice of blissid in place of holy as the adjective suggests that "holy Trinite" was not a set phrase in her own mind.
An 1841 edition of Lyttleton, His Treatise of Tenures, in French and English includes in its introduction the text of Thomas Lyttleton's will (written not later than 1481), which includes this provision:
This is Julian of Norwich's preferred phrase with a vengeance (especially as Lyttleton charges that the Lord of Frankley must meet the terms of this bequest and obligation "as he will answer to the blessed Trinitë").
However, a search of the Early English Text Society's collection, Songs, Carols, and Other Miscellaneous Poems, from the Balliol Ms. 354, Richard Hill's Commonplace-book (1907), whose contents Hill gradually collected over the period 1508–1536, finds one crucial match. Searches within the book yield 43 pages matches for holy and 12 pages with matches for Trinite [or trinite or Trynite]—and one pair is in combination:
Google Books is very poor and finding matches from before the late 1500s, owing to the relative scarcity of early books in its database and to OCR problems with early English fonts. So there are surely some and perhaps many earlier instances of Holy Trinity in published English than the earliest one it finds (from no later than 1536). But there is at least some reason to doubt that Holy Trinity was already in widespread use in England as holy Trinite in the late medieval period, as well as some reason to suppose that during the period 1390–1490 the wording blessed [or blissid] Trinite may have been in more common use.