Definitions of louse:

• noun:   wingless usually flattened blood-sucking insect parasitic on warm-blooded animals
• noun:   any of several small insects especially aphids that feed by sucking the juices from plants
• noun:   a person who has a nasty or unethical character undeserving of respect
• noun:   wingless insect with mouth parts adapted for biting; mostly parasitic on birds