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").
there is mischief afoot
plans are afoot
she was afoot when I saw her this morning
they went to the village afoot
quail are hunted either afoot or on horseback