late 14c., legacie, "body of persons sent on a mission," from Medieval Latin legatia, from Latin legatus "ambassador, envoy, deputy," noun use of past participle of legare "send with a commission, appoint as deputy, appoint by a last will" (see legate).
Sense of "property left by will, a gift by will" appeared in Scottish mid-15c. Legacy-hunter is attested from 1690s. French legs "a legacy" is a bad spelling of Old French lais (see lease (n.)). French legacie is attested only from 16c.