The Cupid's bow or tubercle 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 inVenus and Adonis:
For pity now she can no more detain him; (577)
The poor fool prays her that he may depart:
She is resolv'd no longer to restrain him,
Bids him farewell, and look well to her heart, (580)
The which, by Cupid's bow she doth protest Extract From Shakespeare.