Lappen.

From Middle High German lappe, from Old High German lappo, from Proto-West Germanic *lappā (“cloth, rag, lobe”) of uncertain origin, possibly Proto-Indo-European *leb- (“to hang loosely”). The form is remarkable because of the unshifted geminate -pp- (instead of expected Old High German *lapfo). Based on the supposedly invariable nature of the consonant shift, lappo would have to be an originally Central and Low German form (compare Old Saxon lappo). This is, however, quite unlikely since the Old High German word is attested as early as the 9th century, and no variants with -pf- exist. The form remains thus unexplained.