type of annual leguminous plant, also its edible seed, mid-13c., from Old French lentille "lentil," also "a freckle" (12c.), from Latin lenticula, diminutive of Latin lens (genitive lentis) "lentil plant, a lentil," cognate with Greek lathyros "pulse;" Old High German German linsa, German linse "a lentil;" Old Church Slavonic lęšta, Russian ljač.
The similarity between Slavic, Gm. and Latin seems too great to be coincidental, but a common preform cannot be reconstructed. Like other agricultural terms, 'lentil' may have been borrowed from a non-IE language in Europe. [de Vaan]