"lips forever painted in the petulant cupid's bow of her youth"

Cupid
Public DomainCupid - Credit: Léon Bazille Perrault
"cupid's bow" refers to the shape of the upper lip, where the double curve resembles the bow of Cupid, the Roman god of love.

 

Cupid's Bow
Creative Commons AttributionCupid's Bow - Credit: Francisca Ulloa