The Collaborative International Dictionary of English v.0.48:

Water locust \Wa"ter lo"cust\ (Bot.)
   A thorny leguminous tree (Gleditschia monosperma) which
   grows in the swamps of the Mississippi valley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water locust
    n 1: honey locust of swamps and bottomlands of southern United
         States having short oval pods; yields dark heavy wood [syn:
         water locust, swamp locust, Gleditsia aquatica]