Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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