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.
mid-15c., "one to whom any return is due or payable, one to whom money is owed," from Anglo-French creditour, Old French creditor (early 14c.), from Latin creditor "truster; lender," from creditus, past participle of credere "to believe" (see credo).
"person to whom a privilege or concession has been granted," 1848, from French concessionaire "person to whom a concession has been granted," from concession, from Latin concessionem "an allowing" (see concession). Native form concessionary is attested from 1854; American English concessioner is from 1899.