c.1300, from Old French sauvete, earlier salvetet (11c.), from Medieval Latin salvitatem (nominative salvitas) "safety," from Latin salvus (see safe). Meaning "trigger-lock on a gun" is attested from 1881. As a North American football position, first recorded 1881.
Safety-pin is from 1857; safety-valve is from 1797; figurative sense recorded from 1818. Safety-net in literal sense (in machinery) by 1916, later of aerial circus performances (1920s); figurative use by 1950. Safety-first as an accident-prevention slogan first used in Britain in 1873 (said to be originally from U.S. railroads); widely used on Conservative Party election posters in 1922.