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.

also wood-louse, 1610s, from wood (n.) + louse (n.). So called from being found in old wood.

"clear of lice," 1918, from de- + louse (n.). First in reference to World War I armies. Related: Deloused; delousing.

mid-14c., lousi, "infested with lice," from louse (n.) + -y (2). Figurative use as a generic adjective of abuse dates from late 14c.; sense of "swarming with" (money, etc.) is American English slang from 1843. Related: Lousiness.

"infested with lice, lousy; pertaining to lice," 1540s, from Latin pediculosus, from pediculus "louse" (see pediculosis). Related: Pedicular "of or pertaining to a louse or lice" (1650s), from Latin pedicularis, from pediculus.

"louse egg," Middle English nite, from Old English hnitu, from Proto-Germanic *hnitu- (source also of Norwegian nit, Middle Dutch nete, Dutch neet, Middle High German niz, German Niß), from PIE root *knid- "egg of a louse" (source also of Russian, Polish gnida, Czech knida; Greek konis, genitive konidos "egg of a louse").

"lice infestation," 1809, with -osis + Latin pediculus, diminutive of pedis "a louse," said in some sources to be akin to pedere "to break wind" (see petard) on notion of "foul-smelling insect" [Watkins]. But de Vaan traces it to a PIE *pesd- "annoying insect" and compares Avestan pazdu- "beetle, maggot." Pedicule "louse" is attested in Middle English (early 15c.).

"body louse," 1917, British World War I slang, earlier in nautical use, said to be from Malay (Austronesian) kutu, the name of some parasitic, biting insect.

also pill-bug, kind of wood-louse or other insect-like crustacean which can roll itself into a ball like a pill, 1841, from pill (n.) + bug (n.).