late 14c., "serving to effect the intended purpose," from Old French effectif, from Latin effectivus "productive, effective," from effect-, stem of efficere "work out, accomplish" (see effect (n.)). Of military forces, "fit for action or duty," from 1680s.
"producing an effect; having power to produce an effect," late 14c., Old French effectuel, from Late Latin effectualis, from Latin effectus "accomplishment, performance" (see effect (n.)). Used properly of actions (not agents) and with a sense "having the effect aimed at" (effective, by contrast, is used of the agent or the thing done and with a sense "having great effect"). Related: Effectually; effectualness.
"tried, tested; experienced, expert; reliable, effective, trustworthy," late 14c., past-participle adjective from approve (v.).
late 14c., "making, producing immediate effect, active, effective," from Old French efficient and directly from Latin efficientem (nominative efficiens) "effective, efficient, producing, active," present participle of efficere "work out, accomplish," from assimilated form of ex "out" (see ex-) + facere "to do" (from PIE root *dhe- "to set, put"). Meaning "productive, skilled" is from 1787. Related: Efficiently.