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."
"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.