Know how to use "fewer" and "less"? Find out.
1846, from Irish Sile na gcioch, literally "Sheila of the breasts" [OED]. According to modern folklorists, not a Celtic survival, but originating rather in the Romanesque churches of France and northern Spain. Their theories that it is meant to degrade the female body and discourage sexuality, or that it is meant as an apotropaic gesture to ward off the devil, are not entirely convincing.