The Collaborative International Dictionary of English v.0.48:

Panic \Pan"ic\, n. [L. panicum.] (Bot.)
   A plant of the genus Panicum; panic grass; also, the edible
   grain of some species of panic grass.
   [1913 Webster]

   Panic grass (Bot.), any grass of the genus Panicum.
      [1913 Webster]

WordNet (r) 3.0 (2006):

panic grass
    n 1: any grass of the genus Panicum; grown for grain and fodder