Any one of numerous species of small, wingless, suctorial, parasitic insects belonging to a tribe (Pediculina), now usually regarded as degraded Hemiptera. To this group belong of the lice of man and other mammals; as, the head louse of man (Pediculus capitis), the body louse (P. vestimenti), and the crab louse (Phthirius pubis), and many others. See Crab louse, Dog louse, Cattle louse, etc., under Crab, Dog, etc.
- Any of numerous small, flat-bodied, wingless biting or sucking insects of the orders Mallophaga or Anoplura, many of which are external parasites on various animals, including humans.
- Slang A mean or despicable person.
- Slang To bungle: loused the project; louse up a deal.