The Collaborative International Dictionary of English v.0.48:

Cack \Cack\ (k[a^]k), v. i. [OE. cakken, fr. L. cacare; akin to
   Gr. kakka^n, and to OIr. cacc dung; cf. AS. cac.]
   To ease the body by stool; to go to stool. --Pope.
   [1913 Webster]