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"parasitic insect infecting human hair and skin," Old English lus, from Proto-Germanic *lus (cf. Old Norse lus, Middle Dutch luus, Dutch luis, Old High German lus, German Laus), from PIE *lus- "louse" (cf. 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.
n. pl. lice (līs)
Any of numerous small, flat-bodied, wingless biting or sucking insects of the orders Mallophaga or Anoplura, many of which are external parasites on humans.
any of about 2,900 species of small, wingless insects (order Phthiraptera), worldwide in distribution, that have chewing mouthparts, a flattened body, and shortened front legs used to transport food to the mouth. Chewing lice may be from 1 to 5 mm (0.039 to 0.19 inch) in length, and their colour ranges from white to black. The life cycle is spent on the feathers or hair of the host, though one genus lives in the throat pouches of pelicans and cormorants.