The Collaborative International Dictionary of English v.0.48:

Avast \A*vast"\ ([.a]*v[.a]st"), interj. [Corrupted from D. houd
   vast hold fast. See Hold, v. t., and Fast, a.] (Naut.)
   Cease; stop; stay. "Avast heaving." --Totten.
   [1913 Webster]