zealous (adj.)

"full of zeal" (in the service of a person or cause), 1520s, from Medieval Latin zelosus "full of zeal" (source of Italian zeloso, Spanish celoso), from zelus (see zeal). The sense "fervent, inspired" was earlier in English in jealous (late 14c.), which is the same word but come up through French. Related: Zealously, zealousness.