The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Bot.) Any plant of the labiate genus Scutellaria, the
      calyx of whose flower appears, when inverted, like a
      helmet with the visor raised.
      [1913 Webster]

   3. (Zool.) The Lophiomys.
      [1913 Webster]

   Mad-dog skullcap (Bot.), an American herb (Scetellaria
      lateriflora) formerly prescribed as a cure for
      hydrophobia.
      [1913 Webster]

WordNet (r) 3.0 (2006):

skullcap
    n 1: rounded brimless cap fitting the crown of the head
    2: a herbaceous plant of the genus Scutellaria which has a calyx
       that, when inverted, resembles a helmet with its visor raised
       [syn: skullcap, helmetflower]
    3: the dome of the skull [syn: calvaria, skullcap]