early 15c., in law, "the senior justices of the peace," whose presence was necessary to constitute a court, from Latin quorum "of whom," genitive plural (masc. and neuter; fem. quarum) of qui "who" (from PIE root *kwo-, stem of relative and interrogative pronouns).
The traditional wording of the commission appointing justices of the peace translates as, "We have also assigned you, and every two or more of you (of whom [quoram vos] any one of you the aforesaid A, B, C, D, etc. we will shall be one) our justices to inquire the truth more fully." The justices so-named usually were called the justices of the quorum.
Meaning "fixed number of members of any constituted body whose presence at a particular meeting is necessary to transact business" is recorded by 1610s.
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.