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)

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "hassock":
   Eames chair, Hitchcock chair, TV chair, armchair, armless chair,
   back seat, banquette, bar stool, barber chair, barrel chair,
   basket chair, batch, batwing chair, bed chair, bench, bicycle seat,
   boudoir chair, bow-back chair, bucket seat, bunch, camp chair,
   campaign chair, club chair, club lounge chair, clump, cluster,
   comb-back chair, contour chair, copse, cricket, cricket chair,
   crop, dining chair, draft chair, easy chair, elbowchair,
   fan-back chair, fauteuil, fender stool, folding chair, foldstool,
   form, garden chair, group, grouping, groupment, grove, growth,
   high chair, horse, knot, ladder-back chair, lot, mess,
   milking stool, ottoman, overstuffed chair, pew, plantation,
   planting, platform rocker, rocker, rocking chair, sedan chair,
   settle, shock, slew, stand, step stool, stook, stool, swing,
   taboret, thicket, throne, tuft, tussock, wisp