mossback

1. An old shellfish or turtle with a growth of algae on its back.
2. An old, large, or sluggish fish.
3. An extremely conservative or old-fashioned person.

noun

1. A person who vehemently, often fanatically opposes progress and favors return to a previous condition: die-hard, reactionary, ultraconservative. See politics.
2. An old-fashioned person who is reluctant to change or innovate: fogy, fossil, fuddy-duddy. Informal stick-in-the-mud. Slang square. See new/old.

The noun has one meaning:

Meaning #1: an elderly man
  Synonyms: old-timer, oldtimer, gaffer, old geezer, antique