Chapter Fons et Origo to Foot of F by Brewer's Phrase & Fable

foot in it nicely. You have got yourself into a pretty mess. (In French, vous avez mis le pied dedans.) When porridge is burnt or meat over-roasted, we say, "The bishop hath put his foot in." (See Bishop.)
   Afoot. On the way, in progress. (See Game's Afoot, Matter Afoot.)

"Mischief, thou art afoot,
Take thou what course thou wilt."
Shakespeare: Julias Caesar, iii. 2.