1560s, "violent encounter of armed forces or a pair of warriors," a military term, from Middle French choc "violent attack," from Old French choquer "strike against," probably from Frankish, from a Proto-Germanic imitative base (compare Middle Dutch schokken "to push, jolt," Old High German scoc "jolt, swing").
Meaning "a sudden blow" is from 1610s; meaning "a sudden and disturbing impression on the mind" is from 1705. Sense of "feeling of being (mentally) shocked" is from 1876. Medical sense is attested from 1804 (it also once meant "seizure, stroke," 1794). Shock-absorber is attested from 1906 (short form shocks attested by 1961); shock wave is from 1907. Shock troops (1917) translates German stoßtruppen and preserves the word's original military sense. Shock therapy is from 1917; shock treatment from 1938.