c. 1600, in medicine, from specialized sense of Latin iniectus "a casting on, a throwing over," past participle of inicere "to throw in or on; insert, bring into," from in- "in" (from PIE root *en "in") + -icere, combining form of iacere "to throw" (from PIE root *ye- "to throw, impel"). Related: Injectable; injected; injecting.