An old, cliched excuse that a woman comes up with when she doesn't feel like having sex.
I desparately wanted a blowjob from my girlfriend, but she said she had a headache, so I put powdered aspirin on my dick just to prove that I really needed her to release the building pressure in my balls.
Spare pussy to have when your wife or girlfriend is either on the rag or just not in the mood to straddle the cock. Traditionally a popular stress reliever in France, which might explain why they rarely wage wars these days.
After my wife tried the old "I have a headache" tactic for avoiding sex, I hopped over to the other side of town where my mistress gladly spread her legs for me.