go off (v.) Look up go off at Dictionary.com
1570s, of firearms, etc., "explode, be discharged;" see go (v.) + off (adv.); meaning "depart" is c. 1600; that of "deteriorate in condition" is from 1690s; that of "reprimand" is from 1941 (originally with at, since c. 2000 more often with on).