The Collaborative International Dictionary of English v.0.48:

Thimbleberry \Thim"ble*ber`ry\, n. (Bot.)
   A kind of black raspberry (Rubus occidentalis), common in
   America.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Rubus occidentalis
    n 1: raspberry native to eastern North America having black
         thimble-shaped fruit [syn: black raspberry, blackcap,
         blackcap raspberry, thimbleberry, Rubus occidentalis]