1) A man's personal supply of pleasure-inducing sandwich topping
2) A not-so-very-subtle euphemism for cum.
Craig: "So Bill, what's a big giant fag like you like to have for lunch?"
Bill: "Well, I usually have a big dick sandwich dripping with man mayonnaise."
Semen. Usually considered fairly vulgar.
Before moving out, Carl heaved a bolt of man mayonnaise onto the pillow of his hated room-mate.