What exactly is crabs?

ha what exactly is crabs.? I here people mention it buttt I dont get what exactly it is or where it comes from..I have an idea what its about tho...

crabs- The pubic or crab louse (Phthirus pubis) is a parasitic insect which spends its entire life on human hair and feeds exclusively on blood. Humans are the only known host of this parasite. Humans can also be infested with body lice (Pediculus humanus humanus) and with head lice ( got it off google)