The Collaborative International Dictionary of English v.0.48:

Earthnut \Earth"nut`\, n. (Bot.)
   A name given to various roots, tubers, or pods grown under or
   on the ground; as to:
   (a) The esculent tubers of the umbelliferous plants Bunium
       flexuosum and Carum Bulbocastanum.
   (b) The peanut. See Peanut.
       [1913 Webster]