Afoot

wiktionary

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From a- +‎ foot.

etymonline

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afoot (adv., adj.)

c. 1200, afote, "on foot, walking, not on horseback," contraction of prepositional phrase on fotum; see a- (1) "on" + foot (n.). Meaning "astir, on the move" is from 1520s; figurative sense of "in active operation" is from 1601 ("Julius Caesar").