The Collaborative International Dictionary of English v.0.48:

Simnel \Sim"nel\, n. [OF. simenel cake or bread of wheat flour,
   LL. simenellus wheat bread, fr. L. simila the finest wheat
   flour. Cf. Semolina.]
   1. A kind of cake made of fine flour; a cracknel. [Obs.]
      [1913 Webster]

            Not common bread, but vastel bread, or simnels.
                                                  --Fuller.
      [1913 Webster]

   2. A kind of rich plum cake, eaten especially on Mid-Lent
      Sunday. [Eng.] --Herrick.
      [1913 Webster]

WordNet (r) 3.0 (2006):

simnel
    n 1: a crisp bread of fine white flour
    2: a fruitcake (sometimes covered with almond paste) eaten at
       mid-Lent or Easter or Christmas