early 14c., "to exempt, exonerate, release," from Old French deschargier (12c., Modern French décharger) "to unload, discharge," from Late Latin discarricare, from dis- "do the opposite of" (see dis-) + carricare "load" (see charge (v.)).
Meaning "to unload, to free from" is late 14c. Of weapons, from 1550s. The electrical sense is first attested 1748. Meaning "to fulfill, to perform one's duties" is from c.1400. Related: Discharged; discharging.