The Collaborative International Dictionary of English v.0.48:

Bump \Bump\ (b[u^]mp; 215), v. t. [imp. & p. p. Bumped
   (b[u^]mpt); p. pr. & vb. n. Bumping.] [Cf. W. pwmp round
   mass, pwmpiaw to thump, bang, and E. bum, v. i., boom to
   roar.]
   To strike, as with or against anything large or solid; to
   thump; as, to bump the head against a wall.
   [1913 Webster]