Cupid's bow is a facial feature where the double curve of a human upper lip is said to resemble the bow of Cupid, the Roman god of erotic love. The peaks of the bow coincide with the philtral columns giving a prominent bow appearance to the lip.
The phrase is common in literature, often used related to speech, and therefore the mouth, as in Shakespeare's "Venus and Adonis":[1]
Newborn babies with Fetal Hydantoin Syndrome, a consequence of using Hydantoin and/or its major derivatives Phenytoin, Dantrolene and Fosphenytoin, can present with a Cupid's bow lips, along with other features classically associated with the syndrome.