The Collaborative International Dictionary of English v.0.48:

Sitz bath \Sitz" bath`\ [G. sitzbad.]
   A tub in which one bathes in a sitting posture; also, a bath
   so taken; a hip bath.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sitz bath
    n 1: a bathtub in which your buttocks and hips are immersed as
         if you were sitting in a chair and you bathe in a sitting
         position [syn: sitz bath, hip bath]