A toupee is a partial wig, made to cover a bald patch. You'll have to be careful when you're wearing your toupee on a windy day!
It's easy to joke about toupees, since they often seem to quite obviously be covering up bald spots — especially if the toupee is a slightly different color from the remainder of the wearer's real hair. The noun toupee comes from the French word toupet, "tuft of hair." Originally, a toupee was a decorative tuft of fake hair that was worn for the style, rather than to cover up baldness.