Crabs: Slang for pubic lice, parasitic insects that can infest in the genital area of humans. Pubic lice are usually spread through sexual contact. Rarely, infestation can be spread through contact with an infested person's bed linens, towels, or clothes. The key symptom of pubic lice is itching in the genital area. Lice eggs (nits) or crawling lice can be seen with the naked eye.