The Collaborative International Dictionary of English v.0.48:

Quash \Quash\, v. t. [imp. & p. p. Quashed; p. pr. & vb. n.
   Quashing.] [OF. quasser, F. casser, fr. L. cassare to
   annihilate, annul, fr. cassus empty, vain, of uncertain
   origin. The word has been confused with L. quassare to shake,
   F. casser to break, which is probably of different origin.
   Cf. Cashier, v. t.] (Law)
   To abate, annul, overthrow, or make void; as, to quash an
   indictment. --Blackstone.
   [1913 Webster]