The Collaborative International Dictionary of English v.0.48:

Lettish \Let"tish\ (l[e^]t"t[i^]sh), a.
   Of or pertaining to the Letts. -- n. The language spoken by
   the Letts. See Lettic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Lettish
    n 1: the official language of Latvia; belongs to the Baltic
         branch of Indo-European [syn: Latvian, Lettish]