The Collaborative International Dictionary of English v.0.48:

Haystack \Hay"stack`\ (h[=a]"st[a^]k`), n.
   A stack or conical pile of hay in the open air.
   [1913 Webster]

WordNet (r) 3.0 (2006):

haystack
    n 1: a stack of hay [syn: haystack, hayrick, rick]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "haystack":
   anthill, bank, cock, drift, dune, embankment, haycock, haymow,
   hayrick, heap, hill, molehill, mound, mow, pile, pyramid, rick,
   snowdrift, stack