Source : Webster's Revised Unabridged Dictionary (1913)

Afoul \A*foul"\, adv. & a. [Pref. a- + foul.]
   In collision; entangled. --Totten.

   {To run afoul of}, to run against or come into collision
      with, especially so as to become entangled or to cause
      injury.

Source : WordNet®

afoul
     adj : especially of a ship's lines etc; "with its sails afoul"; "a
           foul anchor" [syn: {afoul(ip)}, {foul}, {fouled}]