The building blocks of English

Affixes

-pagus

Conjoined twins.

Greek pagos, something firmly fixed.

This ending is used in a few medical terms for ways in which twins can be born joined together: thoracopagus, joined at the thorax, craniopagus, at the head, pygopagus (Greek pugē, rump) at the buttocks or lower back, and xiphopagus (Greek xiphos, sword), at the xiphoid process, the cartilaginous section at the lower end of the breastbone.