Mid 17th century (in an earlier sense). From post-classical Latin operatorium workshop, use as noun of neuter singular of operatorius operatory.

Mid 16th century; earliest use found in Thomas Cranmer (1489–1556), archbishop of Canterbury. From post-classical Latin operatorius efficacious (Vetus Latina) from classical Latin operāt-, past participial stem of operārī + -ōrius. With operatory word compare post-classical Latin verbum operatorium (Vetus Latina).