When your nipples get hard.

When your nipples become hard and stick out.

A nipple boner is usually a small foreign penile protruding from a woman's breast; Usually milky in color and it smells like shat.

Very "out coming" or "outward" nipples usually on males

When females get boner nipples is called nipping; when females get boner nipples it's called a nipping

Erection in the nipples