The Collaborative International Dictionary of English v.0.48:

Hassock \Has"sock\ (h[a^]s"s[u^]k), n. [Scot. hassock, hassik, a
   besom, anything bushy, a large, round turf used as a seat,
   OE. hassok sedgy ground, W. hesgog sedgy, hesg sedge, rushes;
   cf. Ir. seisg, and E. sedge.]
   1. A rank tuft of bog grass; a tussock. --Forby.
      [1913 Webster]

   2. A small stuffed cushion or footstool, for kneeling on in
      church, or for home use.
      [1913 Webster]

            And knees and hassocks are well nigh divorced.
      --Cowper.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hassock
    n 1: thick cushion used as a seat [syn: ottoman, pouf,
         pouffe, puff, hassock]
    2: a cushion for kneeling on (as when praying in church)