When a man has sex with a woman on her period, then she sucks his penis, then they kiss. The blood travels from her vagina to his penis back to her mouth then finally to his mouth, making a large Z.

when you're hungover as shit after a spending a night drunk as a skunk

When a guy is fucking a girl from behind anally. He pulls his cock out of the girls arse and it drags across her arse cheak, and because there is a small amount shit on the tip of his penis, it leaves smeared lines across her arse similar to the lines left by zorro's sword.

giving a girl back door action then writing the letter z on her belly with your wand.

When a man pulls out as he is ejaculating during intercourse and quickly shapes a letter Z on the womans stomach with his semen while loudly yelling,"Zorro!" with a spanish accent.