Skullcap \Skull"cap`\, n. 1. A cap which fits the head closely; also, formerly, a headpiece of iron sewed inside of a cap for protection. 2. (Bot.) Any plant of the labiate genus {Scutellaria}, the calyx of whose flower appears, when inverted, like a helmet with the visor raised. 3. (Zo["o]l.) The Lophiomys. {Mad-dog skullcap} (Bot.), an American herb ({Scetellaria lateriflora}) formerly prescribed as a cure for hydrophobia.