An Etymological Dictionary of the German Language/Annotated/Linse
Linse, feminine, ‘lentil, lens,’ from the equivalent Middle High German linse, Old High German linsi, feminine, with the Middle High German and Old High German variant linsîn. It is not certain whether the word comes from Latin lens, feminine, because other borrowed terms are based not on the nominative of the Latin word (compare Kreuz, Kelch, yet also Pabst), but on the stem appearing in the oblique cases; hence Latin lent- (as is shown by Anglo-Saxon lens) ought to have appeared as *linz- in High German. An analogous case of an apparent permutation of nt to ns is furnished by English flint, equivalent to Old High German flins, Middle High German vlins (see Flinte); these difficulties are not yet solved. Erbse, however, testifies that we are not compelled to assume that Linse was borrowed from Latin. Compare also Old Slovenian lęšta (from *lentja), Lithuanian lènszis, ‘lentil.’