Etymology
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fin de siecle (adj.)

1890, from French fin de siècle "end of century," phrase used as an adjective. At the time it meant "modern;" now it means "from the 1890s." "App. first in title of a comedy, Paris fin de siècle, produced at the Gymnase, Feb. 1890" [Weekley]. French siècle "century, age" is from Latin saeculum "age, span of time, generation" (see secular).

No proof is needed of the extreme silliness of the term. Only the brain of a child or of a savage could form the clumsy idea that the century is a kind of living being, born like a beast or a man, passing through all the stages of existence, gradually ageing and declining after blooming childhood, joyous youth, and vigorous maturity, to die with the expiration of the hundredth year, after being afflicted in its last decade with all the infirmities of mournful senility. [Max Nordau, "Degeneration," English translation, 1895]
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si 

"yes" in Italian and Spanish; in both instances from Latin sic "so" (see sic).

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fin (n.)
Old English finn "fin," from Proto-Germanic *finno (source also of Middle Low German vinne, Dutch vin), perhaps from Latin pinna "feather, wing" (see pin (n.)); or, less likely, from Latin spina "thorn, spine" (see spine).

U.S. underworld slang sense of "$5 bill" is 1925, from Yiddish finif "five," from German fünf (from PIE root *penkwe- "five") and thus unrelated. The same word had been used in England in 1868 to mean "five pound note" (earlier finnip, 1839).
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de 

Latin adverb and preposition of separation in space, meaning "down from, off, away from," and figuratively "concerning, by reason of, according to;" from PIE demonstrative stem *de- (see to). Also a French preposition in phrases or proper names, from the Latin word.

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de- 

active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.

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de jure 

Latin, literally "of law," thus "legitimate, lawful, by right of law, according to law." Jure is ablative of ius "law" (see de +  just (adj.)).

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de rigueur 
1849, French, literally "of strictness," thus "according to obligation of convention." See rigor.
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de minimis 
Latin, literally "of little things," thus, "so minor as to not be worth regarding."
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de-accession (v.)

also deaccession, "remove an entry for an item from the register of a museum, library, etc." (often a euphemism for "to sell"), by 1968, from de- "off, away" + accession, which had been used since 1887 in library publications as a verb meaning "to add to a catalogue." Related: De-accessioned; de-accessioning.

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de-bamboozle (v.)

"undeceive, disabuse," 1919; see de- "do the opposite of" + bamboozle.

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