1743, from Greek ptosis, literally "falling, a fall," also "the case of a noun," nominal derivative of piptein "to fall" (see symptom). In English, especially of the eyelid. Related: Ptotic.
ptosis pto·sis (tō'sĭs)n. pl. pto·ses (-sēz) Abnormal lowering or drooping of an organ or a part, especially a drooping of the upper eyelid caused by muscle weakness or paralysis.