action n.

mid-14c., accioun, "cause or grounds for a lawsuit," from Anglo-French accioun, Old French accion, action (12c.) "action; lawsuit, case," from Latin actionem (nominative actio) "a putting in motion; a performing, a doing; public acts, official conduct; lawsuit, legal action" (source also of Spanish accion, Italian azione), noun of action from past-participle stem of agere "to do" (from PIE root *ag- "to drive, draw out or forth, move").

Spelling with the restored Latin -t- begins in 15c. The meaning "active exertion, activity" is from late 14c. The sense of "something done, an act, deed" is late 14c. The meaning "military fighting" is from 1590s. The meaning "way in which (a firearm, etc.) acts" is from 1845. As a film director's command, it is attested from 1923.

The meaning "noteworthy or important activity" in a modern sense by 1933, as in the figurative phrase a piece of the action (by 1965), perhaps from a sense of action in card-playing jargon attested by 1914.

No "action" can be had on a bet until the card bet upon appears. If it does not appear after a turn has been made, the player is at liberty to change his bet, or to remove it altogether. Each bet is made for the turn only, unless the player chooses to leave it until he gets some action on it. [from "Faro" in "Hoyle's Games," A.L. Burt Company, New York: 1914]

But there are uses of action as far back as c. 1600 that seem to mean "noteworthy activity." The meaning "excitement" is recorded from 1968. In action "in a condition of effective operation" is from 1650s. Phrase actions speak louder than words is attested from 1731. Action-packed is attested from 1953, originally of movies.