The Collaborative International Dictionary of English v.0.48:

Rattlebox \Rat"tle*box`\ (r[a^]t"t'l*b[o^]ks`), n.
   1. A toy that makes a rattling sound; a rattle.
      [1913 Webster]

   2. (Bot.)
      (a) An American herb (Crotalaria sagittalis), the seeds
          of which, when ripe, rattle in the inflated pod.
      (b) Any species of Crotalaria, a genus of
          yellow-flowered herbs, with inflated, many-seeded
          pods.
          [1913 Webster]

WordNet (r) 3.0 (2006):

rattlebox
    n 1: any of various plants of the genus Crotalaria having
         inflated pods within which the seeds rattle; used for
         pasture and green-manure crops [syn: crotalaria,
         rattlebox]