The Collaborative International Dictionary of English v.0.48:

Begnaw \Be*gnaw"\, v. t. [p. p. Begnawed, (R.) Begnawn.]
   [AS. begnagan; pref. be- + gnagan to gnaw.]
   To gnaw; to eat away; to corrode.
   [1913 Webster]

         The worm of conscience still begnaw thy soul. --Shak.
   [1913 Webster]