The Collaborative International Dictionary of English v.0.48:

Prosit \Pro"sit\, interj. [L., 3d pers. sing. subj. present of
   prodesse to do good; pro for + esse to be.]
   Lit., may it do (you) good; -- a salutation used in well
   wishing, esp. among Germans, as in drinking healths; -- also
   used in the contracted form prost.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Prost \Prost\, interj. [L., see prosit.]
   a contracted form of prosit.
   [PJC]