Stealth

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Middle English (in the sense ‘theft’): probably representing an Old English word related to steal, + -th2.

wiktionary

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From Middle English stelthe, from Old English stǣlþ, from Proto-Germanic *stēliþō [1], equivalent to steal +‎ -th. Compare Old English stalu(“theft, stealth”), Old High German stāla(“theft”), German Diebstahl(“theft”).

etymonline

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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.