A person with horrendously lacking personal hygiene. Most crabmen, or crabbies, tend to have to long greasy hair, neckbeards, excessive amounts of fat and hubris. The best way to identify a crabman from a disgusting person is the noxious aroma surrounding said crabby, often having a radius of 3 meters or more.
Daaaaaaaaaaaaaaaaaaamn, son, look at that crabman over there! I can smell him from across the hallway!