Your noggin is your head. If you're not paying attention during a volleyball game, the ball might hit you right on the noggin.
Your goofy uncle might say, "Let's put our noggins together and think of something fun to do today!" He means your brains, or your heads, of course. Noggin is an informal, slightly silly way to say "head." Its origin is a complete mystery, and it's only had this meaning in American English since the 1860s. Earlier, noggin was slang for "a small cup, mug, or drink."