sine qua non

c. 1300, sauns, saun, "without" (mid-12c. in surnames), from Old French san, sans, sen, senz (some of the forms with adverbial genitive -s) "without, except, apart, not counting." This is cognate with Provençal senes, Old Catalan senes, Old Spanish sen (Spanish sin), Old Italian sen, all from Vulgar Latin *sene, from Latin sine "without," an enlarged form of sed, se "without" (from PIE root *sen(e)- "apart, separated;" see sunder).

"A French word which has existed long in English without becoming naturalized; now archaic or affected, except as used in heraldry ..." [Century Dictionary, 1891]; OED writes that the words limited modern use is "chiefly with reminiscence of Shakespere," which it spells that way. In reference to fonts, by 1927, short for sans-serif. Sans souci, French, as an adverbial phrase "free from care, without care or concern," was the name of Frederick the Great's royal palace at Potsdam. 

also *kwi-, Proto-Indo-European root, stem of relative and interrogative pronouns.

It forms all or part of: cheese (n.2) "a big thing;" cue (n.1) "stage direction;" either; hidalgo; how; kickshaw; neither; neuter; qua; quality; quandary; quantity; quasar; quasi; quasi-; query; quib; quibble; quiddity; quidnunc; quip; quodlibet; quondam; quorum; quote; quotidian; quotient; ubi; ubiquity; what; when; whence; where; whether; which; whither; who; whoever; whom; whose; why.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit kah "who, which;" Avestan ko, Hittite kuish "who;" Latin quis/quid "in what respect, to what extent; how, why," qua "where, which way," qui/quae/quod "who, which;" Lithuanian kas "who;" Old Church Slavonic kuto, Russian kto "who;" Old Irish ce, Welsh pwy "who;" Old English hwa, hwæt, hwær, etc.

a prefix used freely in English and meaning "not, lack of," or "sham," giving a negative sense to any word, 14c., from Anglo-French noun-, from Old French non-, from Latin non "not, by no means, not at all, not a," from Old Latin noenum "not one" (*ne oinom, from PIE root *ne- "not" + PIE root *oi-no- "one, unique"). In some cases perhaps from Middle English non "not" (adj.), from Old English nan (see not). "It differs from un- in that it denotes mere negation or absence of the thing or quality, while un- often denotes the opposite of the thing or quality" [Century Dictionary].