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Webster 1913 Edition

Noun.

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.]

mossback ‎(plural mossbacks)

A turtle that, because of its age, has a growth of algae on its back.
(by extension) A very conservative or reactionary person, especially one with old-fashioned views.

turtle with a growth of algae

very conservative or reactionary person