WordNet (r) 3.0 (2006):

treble damages
    n 1: three times the amount that a court would normally find the
         injured party entitled to

Bouvier's Law Dictionary, Revised 6th Ed (1856):

TREBLE DAMAGES, remedies. In actions arising ex contractu some statutes give
treble damages; and these statutes have been liberally construed to mean
actually treble damages; for example, if the jury give twenty dollars
damages for a forcible entry the court will award forty dollars more, so as
to make the total amount of damages sixty dollars. 4 B. & C. 154; McClell.
Rep. 567.
     2. The construction on the words treble damages, is different from that
which has been put on the words treble costs. (q.v.) Vide 6 S. & R. 288; 1
Browne, R. 9; 1 Cowen, R. 160, 175,176, 584; 8 Cowen, 115.