stealth (n.)

mid-13c., "theft, action or practice of stealing," from a probable Old English *stælþ, which is related to stelen (see steal (v.)), from Proto-Germanic *stælitho (source also of Old Norse stulþr), with Proto-Germanic abstract noun suffix *-itho (see -th (2)).

Compare heal/health, weal/wealth. Sense of "secret action" developed c. 1300, but the word also retained its etymological sense into 18c. Got a boost as an adjective from stealth fighter, stealth bomber, radar-evading U.S. military aircraft, activated 1983.