louse

English louse comes from Middle English lous, from Old English lūs, from Proto-West Germanic *lūs, from Proto-Germanic *lūs, ultimately from Proto-Indo-European *lewH-.

The word has stayed remarkably close to its ancient form and meaning, moving from Old English lūs to Middle English lous with barely a shift in sound or sense.

It's cognate with Dutch luis, German Laus, and the Scandinavian forms lus, all descending from the same Germanic root — and possibly related to Welsh llau and Sanskrit यूका (yūkā).