Etymology
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pro tanto 

Latin, literally "for so much; to such an extent," from pro "for, so far as" (see pro-) + ablative singular neuter of tantus "so much," from tam "so" (see tandem).

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per diem 

1510s, "by the day, in each day," Latin, "by the day," from per (see per) + diem, accusative singular of dies "day" (from PIE root *dyeu- "to shine"). As a noun from 1809, "amount or allowance of so much every day."

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in memoriam 

Latin, literally "in memory of," from accusative of memoria "memory" (from PIE root *(s)mer- (1) "to remember"). The phrase was much-used in Latin writing; Tennyson's poem of that name (published in 1850) seems to have introduced the phrase to English.

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free verse (n.)

1869; Englishing of vers libre.

But it is possible that excessive devotion to rhyme has thickened the modern ear. The rejection of rhyme is not a leap at facility; on the contrary, it imposes a much severer strain upon the language. When the comforting echo of rhyme is removed, success or failure in the choice of words, in the sentence structure, in the order, is at once more apparent. Rhyme removed, the poet is at once held up to the standards of prose. Rhyme removed, much ethereal music leaps up from the word, music which has hitherto chirped unnoticed in the expanse of prose. [T.S. Eliot, "Reflections on Vers libre," New Statesman, 3 March 1917]
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ad lib 

also ad lib., 1811 as a musical instruction, shortened from Latin ad libitum "to (one's) pleasure, as much as one likes" (c. 1600), from ad "to" (see ad-) + libitum "pleasure," accusative of libere "to please" (see libido). As a noun from 1825; as a verb by 1915.

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birthday suit (n.)
first attested 1730s, but probably much older. The notion is the suit of clothes one was born in, i.e., no clothes at all. Compare Middle English mother naked "naked as the day one was born;" Middle Dutch moeder naect, German mutternackt.
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vers libre (n.)

1902, from French, literally "free verse," lines of varying length.

I remarked some years ago, in speaking of vers libre, that 'no vers is libre for the man who wants to do a good job.' The term, which fifty years ago had an exact meaning in relation to the French alexandrine, now means too much to mean anything at all. [T.S. Eliot, introduction to "Selected Poems of Ezra Pound," 1928]
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vice versa 

"the order being changed," c. 1600, Latin, from vice, ablative of vicis "a change, alternation, alternate order" (from PIE root *weik- (2) "to bend, to wind") + versa, feminine ablative singular of versus, past participle of vertere "to turn, turn about" (from PIE root *wer- (2) "to turn, bend"). "The phrase has the complete force of a proposition, being as much as to say that upon a transposition of antecedents the consequents are also transposed" [Century Dictionary].

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comme il faut 

"according to etiquette," 1756, French, literally "as it should be." From comme "as, like, how," from Old French com, from Vulgar Latin *quomo, from Latin quomodo "how? in what way?," pronominal adverb of manner, related to quam "how much?," qui "who" (from PIE root *kwo-, stem of relative and interrogative pronouns). With il, from Latin ille "this" (see le) + faut, third person singular present indicative active of falloir "be necessary," literally "be wanting or lacking" (see fail (v.)).

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white trash (n.)

1824, originally in African-American vernacular in the South.

The slaves themselves entertain the very highest contempt for white servants, whom they designate as 'poor white trash.' [Fanny Kemble, journal, Jan. 6, 1833]
[T]he term [poor white] is rather loosely applied by Northern writers even to mountaineers and to small farmers who live on a precarious footing. But in the Southern conception, not everyone who is both poor and white is a "poor white." To the Southerner, the "poor white" in the strictest sense is a being beyond the pale of even the most generous democratic recognition; in the negro's term, "po' white trash," or so much social débris. [Robert Penn Warren, "The Briar Patch," 1930, footnote]
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