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, bequeath," from lex (genitive legis) "contract, law" (see legal). General sense of "ambassador, delegate, messenger" is from late 14c.
official who acted as a deputy general to governors of provinces conquered by ancient Rome in the 2nd and 1st centuries BC, during the period of the republic. In the latter part of the 1st century BC, Julius Caesar initiated the practice of appointing legates to command legions in the army. This practice became customary under the emperor Augustus (27 BC-AD 14). Under the early empire, in the 1st and 2nd centuries AD, a province containing one or more legions was governed by a military commander with the title legatus Augusti pro praetore (propraetorian legate of the emperor).