What does a lily tattoo symbolize?

A lily tattoo is symbolic of fertility and motherhood. The meaning is derived from representations of the lily in Greek mythology, with legends associating the creation of the flower with the breast milk of the wife of Zeus — Hera.

According to Greek lore, Zeus wished for Hercules (his illegitimate son) to become a god by suckling the breast of Hera, so he put Hera to sleep in order for Hercules to drink her milk. Hercules' vigorous suckling caused an overflow of Hera's milk, with some flowing to the heavens to create the Milky Way and some flowing to earth to form the lily.