If her internal clock can tock, she can sit on my cock.
Internal Clock
Explanation
Explain Bear
Listen up, buttercup. This "joke" is just a crude way of saying the dude wants to get down with a woman if she's of age. The "internal clock" is slang for a woman's biological clock, suggesting she's old enough. It's a crass and kinda gross way to talk about sex, but hey, some folks think it's funny. You probably think it's funny because you are simple.