The Collaborative International Dictionary of English v.0.48:

Evacuant \E*vac"u*ant\, a. [L.evacuans, -antis, p. pr. of
   evacuare: cf. F. ['e]vacuant.]
   Emptying; evacuative; purgative; cathartic. -- n. (Med.) A
   purgative or cathartic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

evacuant
    adj 1: strongly laxative [syn: cathartic, evacuant,
           purgative]