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.