The Collaborative International Dictionary of English v.0.48:

Afoot \A*foot"\, adv. [Pref. a- + foot.]
   1. On foot.
      [1913 Webster]

            We 'll walk afoot a while.            --Shak.
      [1913 Webster]

   2. Fig.: In motion; in action; astir; in progress.
      [1913 Webster]

            The matter being afoot.               --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

afoot
    adv 1: on foot; walking; "they went to the village afoot";
           "quail are hunted either afoot or on horseback"
    adj 1: traveling by foot; "she was afoot when I saw her this
           morning"
    2: currently in progress; "there is mischief afoot"; "plans are
       afoot"; "preparations for the trial are underway" [syn:
       afoot(p), underway]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "afoot":
   accidental, accompanying, ado, afloat, astir, by foot,
   circumstantial, current, doing, eventuating, footback, going on,
   happening, in full swing, in hand, in the wind, incidental,
   occasional, occurring, on, on foot, on footback, ongoing, passing,
   prevailing, prevalent, resultant, stirring, taking place,
   under way