GNU Collaborative International Dictionary of English

Found one definition

1.       
   ‖Prosit , 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.