The Collaborative International Dictionary of English v.0.48:

Seldom \Sel"dom\, a.
   Rare; infrequent. [Archaic.] "A suppressed and seldom anger."
   --Jer. Taylor.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Seldom \Sel"dom\ (s[e^]l"d[u^]m), adv. [Usually, Compar. More
   seldom (m[=o]r" s[e^]l"d[u^]m); superl. Most seldom
   (m[=o]st" s[e^]l"d[u^]m); but sometimes also, Seldomer
   (s[e^]l"d[u^]m*[~e]r), Seldomest.] [AS. seldan, seldon,
   seldum, fr. seld rare; akin to OFries. sielden, D. zelden, G.
   selten, OHG. seltan, Icel. sjaldan, Dan. sielden, Sw.
   s[aum]llan, Goth. sildaleiks marvelous.]
   Rarely; not often; not frequently.
   [1913 Webster]

         Wisdom and youth are seldom joined in one. --Hooker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seldom
    adv 1: not often; "we rarely met" [syn: rarely, seldom]
           [ant: frequently, oft, often, oftentimes,
           ofttimes]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "seldom":
   abnormally, at infrequent intervals, few, hardly, hardly ever,
   incredibly, infrequently, irregularly, little, not often,
   occasional, occasionally, rare, rarely, scarce, scarcely,
   scarcely ever, seldom if ever, sparsely, sporadic, sporadically,
   uncommon, uncommonly, uncustomarily, unexpectedly, unfrequent,
   unfrequently, unnaturally, unoften, unordinarily, unusually,
   very seldom