Penisknee is a photographic optical illusion wherein a photograph's subject appears to have a penis that extends the length of the thigh to the knee. It is a result of a combination of shadows and the contours of the subject's clothing. Penisknee is rare and when properly seen the viewer can experience a Gestalt Switch between perceiving the illusion of the penis and the knee that is actually being viewed.

A portmanteau of carpe diem, a Latin phrase that means “Seize the Day!” and DM, which stands for Direct Messages. This word means seizing the day via DMs.