The phrase "cupid's bow" refers to the shape of a person's upper lip when it appears like the bow of Cupid, the Roman god of love. It describes a prominent, nicely curved, and defined shape resembling the bow that Cupid carries. Full definition
I sweep this over the higher points of my face and down my nose and cupids bow for a beautiful glow that highlights the face in the most flattering of places for a natural finish. (krystelcouture.com)
Tia Semer's 1920s cupids bow lips makeup tutorial — Visit Tia Semer's Eliza cosmetics Facebook page and get the 1920s Theda Bara lip rouge you see featured here! (vintagemakeupguide.com)