mid-14c., "one whose profession is to plead cases in a court of justice," a technical term from Roman law, from Old French avocat "barrister, advocate, spokesman," from Latin advocatus "one called to aid (another); a pleader (on one's behalf), advocate," noun use of past participle of advocare "to call (as witness or adviser), summon, invite; call to aid; invoke," from ad "to" (see ad-) + vocare "to call," which is related to vox (genitive vocis) "voice" (from PIE root *wekw- "to speak").
Also in Middle English as "one who intercedes for another," and "protector, champion, patron." Feminine forms advocatess, advocatrice were in use in 15c.; advocatrix is from 17c.
"plead in favor of," 1640s, from advocate (n.) or from Latin advocatus, past participle of advocare. Related: Advocated; advocating.