The Collaborative International Dictionary of English v.0.48:

Pennyroyal \Pen`ny*roy"al\, n. [A corruption of OE. puliall
   royal. OE. puliall is ultimately derived fr. L. puleium, or
   pulegium regium (so called as being good against fleas), fr.
   pulex a flea; and royal is a translation of L. regium, in
   puleium regium.] (Bot.)
   An aromatic herb (Mentha Pulegium) of Europe; also, a North
   American plant (Hedeoma pulegioides) resembling it in
   flavor.
   [1913 Webster]

   Bastard pennyroyal (Bot.) See Blue curls, under Blue.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Hedeoma pulegioides
    n 1: erect hairy branching American herb having purple-blue
         flowers; yields an essential oil used as an insect
         repellent and sometimes in folk medicine [syn:
         pennyroyal, American pennyroyal, Hedeoma pulegioides]