"requite, repay, or return in kind," 1610s, from Latin retaliatus, past participle of retaliare "pay back in kind," from re- "back" (see re-) + Latin talio "exaction of payment in kind," from or influenced by talis "suchlike" (see that). Originally of kindness, civility, etc., but by 1630s of injury, ill-treatment, etc. (now the usual sense). Intransitive sense is from 1650s. Related: Retaliated; retaliating.