louse n.

parasitic insect infesting human hair and skin, Old English lus, from Proto-Germanic *lus (source also of Old Norse lus, Middle Dutch luus, Dutch luis, Old High German lus, German Laus), from PIE *lus- "louse" (source also of Welsh lleuen "louse").

The meaning "obnoxious person" is from 1630s. The plural lice (Old English lys) shows effects of i-mutation. Grose ["Dictionary of the Vulgar Tongue," 1785] has louse ladder "A stitch fallen in a stocking."

late 14c., "to clear of lice," from louse (n.). Compare delouse. Related: Loused; lousing. To louse up "ruin, botch" first attested 1934, from a literal sense (in reference to bedding), from 1931.