1880 A. H. Sayce Introd. Sci. ofLang. II. vii. 107The old middle or intransitive voice..has been lost in Keltic and Letto-Slavonic. 
1913 L. W. Lyde Continent of Europe 419These are the Letto-Lithuanians and Finnic (Esht) peoples of the Baltic Provinces. 
1920 Glasgow Herald 21 July 8 A Polono-Lithuanian, a Polono-Lettish, and a Letto-Lithuanian question still unsettled. 
1935 Huxley & Haddon We Europeans vii. 221The Letto-Lithuanians tend to be of medium height, but taller where remote from Slav influence. 
ORIGIN: Modern Latin  Letto , from Lett -o-