The Collaborative International Dictionary of English v.0.48:

Soapwort \Soap"wort`\, n. (Bot.)
   A common plant (Saponaria officinalis) of the Pink family;
   -- so called because its bruised leaves, when agitated in
   water, produce a lather like that from soap. Called also
   Bouncing Bet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

soapwort
    n 1: plant of European origin having pink or white flowers and
         leaves yielding a detergent when bruised [syn: soapwort,
         hedge pink, bouncing Bet, bouncing Bess, Saponaria
         officinalis]