The term meaning to drop is ptosis. It can refer to the breast or other body parts e.g the kidney etc. A breast is described as having ptosis when the level at which the nipple lies is BELOW the fold under the breast.

The primary symptom of ptosis is a drooping eyelid.

sinking or drooping

Eyelid ptosis describes a condition where either the upper or lower eyelid droops. Ptosis occurs when the muscles that raise the eyelids become weak.

Ptosis may occur because the levator muscle's attachment to the lid is weakening with age.