The word deadbolt, as applied to a lock, came into use in 1902, according to the Merriam-Webster Dictionary, when it described a bolt that was "dead," rather than "live," meaning a key could unlock it from outside the house.
A Compound Word
"Dead" appears in Old English in the 13th century. "Bolt" came to English from Old High German in about the 12th century as the word "Bolz," which meant a cross bow "bolt" (arrow).
"Dead" came into Old English from Proto-German as "dauthaz" and, earlier from the Indo-European "dheu," meaning "insensible."
The original spelling used the Old English letter "thorn," which disappeared from the language in the transition from Middle to Modern English. Pronounced like the "th" sound in "thick," dead and death were homophones (the same spelling with different meanings).
"Bolt" is a short, stout arrow used in crossbows. The word came from Norse, through Danish, becoming the Proto-Germanic "bultas" and, eventually, the German "Bolz" or "Bolzen"
The modern deadbolt might be a cylindrical slide bolt, or a keyed lock without an opening mechanism, like a knob.