"parasitic insect infecting human hair and skin," Old English lus, from Proto-Germanic *lus (cognates: Old Norse lus, Middle Dutch luus, Dutch luis, Old High German lus, German Laus), from PIE *lus- "louse" (cognates: Welsh lleuen "louse"). Slang meaning "obnoxious person" is from 1630s. The plural lice (Old English lys) shows effects of i-mutation. The verb meaning "to clear of lice" is from late 14c.; to louse up "ruin, botch" first attested 1934, from the literal sense (of bedding), from 1931. Grose ["Dictionary of the Vulgar Tongue," 1785] has louse ladder "A stitch fallen in a stocking."