"act of throwing off or away; refusal to accept or grant," 1550s, from French réjection (16c.) or directly from Latin reiectionem (nominative reiectio) "act of throwing back," noun of action from past-participle stem of reicere (see reject (v.)).
In 19c., it also could mean "excrement." An earlier use was "setting aside of a wife, divorce" (mid-15c.). Medical transplant sense is from 1954. In the psychological sense, relating to parenting, from 1931.