The Collaborative International Dictionary of English v.0.48:

Mossback \Moss"back`\, n.
   A veteran partisan; one who is so conservative in opinion
   that he may be likened to a stone or old tree covered with
   moss. [Political Slang, U.S.]
   [1913 Webster] Mossbanker

WordNet (r) 3.0 (2006):

mossback
    n 1: an extremely old-fashioned conservative

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "mossback":
   Methuselah, antediluvian, antique, back number, backwoodsman,
   bumpkin, clodhopper, conservationist, conservatist, conservative,
   dad, diehard, dodo, elder, fogy, fossil, fud, fuddy-duddy, granny,
   has-been, hayseed, hick, hillbilly, laudator temporis acti,
   longhair, matriarch, mid-Victorian, old believer, old crock,
   old dodo, old fogy, old liner, old man, old poop, old school,
   old woman, old-timer, patriarch, pop, pops, provincial,
   reactionary, regular old fogy, relic, right-winger, rightist,
   square, standpat, standpatter, starets, stick-in-the-mud,
   traditionalist, ultraconservative, yokel