A man's head is his upper dome, as opposed to his penis, which is known just as his dome or in rare cases lower dome. This phrase is used in situations where just saying "dome" by itself could be ambiguous, but it can be used as itself.
Guy1: Your girlfriend rubbed my dome last night, it felt great!
Guy2: What the f...!
Guy1: Chill man, I meant my upper dome.
Guy1: Here take this snap straight to your upper dome.
Guy2: Danks homie.