Origin and history of defuse

"combustible cord or tube for lighting an explosive device," also fuze, 1640s, from Italian fuso, literally "spindle" (the ignition device so called for its shape, because the originals were long, thin tubes filled with gunpowder), from Latin fusus "a spindle," which is of uncertain origin. Influenced by French cognate fusée "spindleful of hemp fiber," and obsolete English fusee "musket fired by a fuse," which is from French. Meaning "device that breaks an electrical circuit" is first recorded 1884, so named for its shape, but erroneously attributed to fuse (v.) because it melts.

active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.