aftershock n.

Old English æfter "behind; later in time" (adv.); "behind in place; later than in time; in pursuit, following with intent to overtake" (prep.), from of "off" (see off (adv.)) + -ter, a comparative suffix; thus the original meaning was "more away, farther off." Compare Old Norse eptir "after," Old Frisian efter, Dutch achter, Old High German aftar, Gothic aftra "behind;" also see aft. Cognate with Greek apotero "farther off," Old Persian apataram "further."

From c. 1300 as "in imitation of." As a conjunction, "subsequent to the time that," from late Old English. After hours "hours after regular working hours" is from 1814. Afterwit "wisdom that comes too late" is attested from c. 1500 but seems to have fallen from use. After you as an expression in yielding precedence is recorded by 1650.

1560s, "violent encounter of armed forces or a pair of warriors," a military term, from 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").

The general sense of "a sudden blow, a violent collision" is from 1610s. The meaning "a sudden and disturbing impression on the mind" is by 1705; the sense of "feeling of being (mentally) shocked" is from 1876.

The electrical sense of "momentary stimulation of the sensory nerves and muscles caused by a sudden surge in electrical current" is by 1746. The medical sense of "condition of profound prostration caused by trauma, emotional disturbance, etc." is by 1804 (it also once meant "seizure, stroke, paralytic shock" 1794).

Shock-absorber is attested from 1906 (short form shocks attested by 1961); shock wave is from 1907. Shock troops (1917), especially selected for assault work, translates German stoßtruppen and preserves the word's original military sense. Shock therapy is from 1917; shock treatment from 1938.