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] MossbankerWordNet (r) 3.0 (2006):
mossback n 1: an extremely old-fashioned conservative