Louse

The AHD defines louse as a noun meaning:

1. pl lice Any of numerous small wingless insects of the orders Mallophaga or Anoplura, many of which are external parasites on various animals, including humans.
2. Slang A mean or despicable person.

It can also be used as a verb with the meaning:

3. Slang To bungle

The word comes through Middle English from the Old English word lus. I liked how the use of the slang word on people can be connected to the insect, in the fact that a human louse is in a way like a parasite on humanity, just like lice are parasites on animals.