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]

The Collaborative International Dictionary of English v.0.48:

mad-dog skullcap \mad-dog skullcap\, mad-dog weed \mad-dog
weed\n. (Bot.)
   An American mint (Scutellaria lateriflora) that yields a
   resinous exudate used esp. formerly as an antispasmodic.

   Syn: blue pimpernel, blue skullcap, Scutellaria
        lateriflora.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mad-dog skullcap
    n 1: an American mint that yields a resinous exudate used
         especially formerly as an antispasmodic [syn: blue
         pimpernel, blue skullcap, mad-dog skullcap, mad-dog
         weed, Scutellaria lateriflora]