Why does "Mayday, Mayday!" mean "help?

Mayday! Mayday!

Why the heck does this mean "help"?

-- At Sea, at sea

It's the English spelling of the pronunciation of the French "M'aider!" "Help me!" Of course, if you've just been mugged in the Metro, the French will know from your accent that you're not French and will wince and sneer and shake their heads at your bad pronunciation and then may or may not come to your rescue.