early 14c., “to exempt, exonerate, release, free (from an obligation),” from Old French deschargier “to unload, discharge” (12c., Modern French décharger), from Late Latin discarricare, from dis- “do the opposite of” (see dis-) + carricare “to load a wagon or cart,” from Latin carrus “two-wheeled wagon” (see car).
Meaning “to fulfill, to perform (one’s duties, etc.)” is from c. 1400. Sense of “dismiss from office or employment” is from c. 1400. Meaning “to unload, to free from, disburden” is late 14c. Of weapons, “send forth by propulsion,” transitive, 1550s; “to fire off,” intransitive, 1580s. Of a river, “to empty itself,” c. 1600. The electrical sense is first attested 1748. Related: Discharged; discharging.