The Collaborative International Dictionary of English v.0.48:

Tref \Tref\ (tr[e^]f), a. [Yiddish, fr. Heb. t[e^]r[=e]ph[=a]h
   an animal torn by wild beasts.]
   Ceremonially unclean, according to the Jewish law; -- opposed
   to kosher.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

tref
    adj 1: not conforming to dietary laws [syn: nonkosher, tref,
           terefah]