The building blocks of English

Affixes

pyg(o)-

The buttocks.

Greek pugē, rump.

A pygidium is the terminal part or hind segment of the body in certain invertebrates; a pygostyle (Greek stulos, column) in a bird is a triangular plate, typically supporting the tail feathers; pygopagus (Greek pagos, fixed) refers to a pair of Siamese twins joined at the buttocks, often back to back.