The Collaborative International Dictionary of English v.0.48:

Prickly ash \Prickly ash\ (Bot.),
   1. A prickly shrub (Xanthoxylum Americanum) with yellowish
      flowers appearing with the leaves; also called toothache
      tree. All parts of the plant are pungent and aromatic.
      The southern species is Xanthoxylum Carolinianum.
      --Gray.
      [1913 Webster]

   2. (Bot.), Hercules'-club, also called the Angelica tree.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Toothache \Tooth"ache`\, n. (Med.)
   Pain in a tooth or in the teeth; odontalgia.
   [1913 Webster]

   Toothache grass (Bot.), a kind of grass (Ctenium
      Americanum) having a very pungent taste.

   Toothache tree. (Bot.)
   (a) The prickly ash.
   (b) A shrub of the genus Aralia (Aralia spinosa).
       [1913 Webster]

WordNet (r) 3.0 (2006):

toothache tree
    n 1: small deciduous aromatic shrub (or tree) having spiny
         branches and yellowish flowers; eastern North America [syn:
         toothache tree, sea ash, Zanthoxylum americanum,
         Zanthoxylum fraxineum]