Afoot means happening or being planned. Something is afoot, plans afoot, mischief afoot.
It nearly always follows its noun — plans are afoot — and it carries a hint of secrecy, which is why the game is afoot became a detective's line. The literal sense is on foot, walking. Note that it is a- plus foot, formed like ahead and asleep.
1adjectiveafootHappening or being planned.
“The children's giggling told her that something was afoot.”
afoot is a state of being under way.
Proto-Indo-European
*ped-
Proto-Indo-European
*pṓds
Proto-Germanic
*fōts
Proto-West Germanic
*fōt
Old English
fōt
Middle English
fot
English
foot
English, today
afoot
From a- + foot. foot comes from Middle English fot, fote, foot, from Old English fōt, from Proto-West Germanic fōt, from Proto-Germanic fōts, from Proto-Indo-European pṓds. a doublet of pes and pie and pous. 2
Etymology from Wiktionary, CC BY-SA