mid-12c., "authorized representative of the Pope," from Old French legat and directly from Latin legatus "ambassador, envoy," originally "provided with a commission," past participle of legare "send as a deputy, send with a commission, charge, bequeath," possibly literally "engage by contract" and related to lex (genitive legis) "contract, law," from PIE root *leg- (1) "to collect, gather." General sense of "ambassador, delegate, messenger of a state or authority" is from late 14c. in English. Related: Legator; legatee; legatine.
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.
However eslegier meant "acquit, clear of charges in a lawsuit," and the Middle English word somehow acquired the meaning of French alléguer, from Latin allegare/adlegare "send for, bring forth, name, produce in evidence, send on business," from ad "to" (see ad-) + legare "to depute, send" (see legate). Related: Alleged; alleging.
historically, "a ruler of a province in the Byzantine Empire;" in the early Church, "a prelate presiding over a diocese;" in the Greek Church, a legate of a patriarch; from Late Latin exarchus, from Greek exarkhos "a leader," from ex (see ex-) + arkhos "leader, chief, ruler" (see archon). Related: Exarchate.