late 14c., "relief from misfortune," see discharge (v.). Meaning "release from work or duty" is from early 15c. Meaning "act of sending out or pouring forth" is from c. 1600; sense of "that which is emitted or poured forth" is from 1727. Meaning "action of firing off a firearm or other missile weapon" is from 1590s. Electricity sense is from 1794.
imaginary animal, coined 1876 by Lewis Carroll in "The Hunting of the Snark." In 1950s, name of a type of U.S. cruise missile, and in 1980s, of a type of sailboat. Meaning "caustic, opinionated, and critical rhetoric" is from c. 2002, probably from snarky and not directly related, if at all, to Lewis Carroll's use of snark.
"to destroy," 1958, probably a back-formation from destruction in the jargon of U.S. aerospace and defense workers to refer to deliberate destruction of a missile in flight by a friendly agent; popularized 1966 in form self-destruct in the voice-over at the beginning of TV spy drama "Mission Impossible." OED records an isolated use of destructed from 17c., in this case probably from Latin destructus, past participle of destruere.