Unknown, likely a borrowing from an unidentified source.
Compare Old High German linsa (“lentil”), Lithuanian lęšis (“lentil”), Proto-Slavic *lęťa (“lentil”), and Albanian lend (“acorn”), sounding too similar for a coincidence, however different enough to prohibit reconstruction of a common PIE protoform. May also be related to Ancient Greek λάθυρος (láthuros).
If ultimately a non-IE substrate loanword, locating the source is virtually impossible because cultivation of lentil was widespread in the region since the Neolithic.