The 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 Venus and Adonis, lines 577–581:
For pity now she can no more detain him;
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, The which, by Cupid's bow she doth protest.