early 15c., rejecten, "eject, set aside, block from inheritance;" late 15c., "refuse to acquiesce or submit to," from Old French rejecter and directly from Latin reiectus, past participle of reiectare "throw away, cast away, vomit," frequentative of reicere "to throw back," from re- "back" (see re-) + -icere, combining form of iacere "to throw" (from PIE root *ye- "to throw, impel").
The meaning "throw away as undesirable or useless, refuse to take for some purpose" is by 1530s. From 1560s as "to repel or rebuff (someone who makes advances of any kind)," especially of a woman refusing a man as a lover or husband (1580s). The sense of "refuse (something offered)" is by 1660s. The medical sense of "show immune response to a transplanted organ" is from 1953. Related: Rejected; rejecting.
mid-15c., "refusal, denial;" 1550s, "a castaway" (both now obsolete), from reject (v.) or obsolete reject (adj.). The sense of "thing cast aside as unsatisfactory" (1893) probably is a fresh extension. Hence "person considered low-quality and worthless" (1925, from use in the militaries in reference to men unsuitable for service).