The Collaborative International Dictionary of English v.0.48:

Butterbur \But"ter*bur`\, n. (Bot.)
   A broad-leaved plant (Petasites vulgaris) of the Composite
   family, said to have been used in England for wrapping up
   pats of butter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

butterbur
    n 1: small Eurasian herb having broad leaves and lilac-pink
         rayless flowers; found in moist areas [syn: butterbur,
         bog rhubarb, Petasites hybridus, Petasites vulgaris]