Hairsplitter

from WordNet (r) 3.0 (2006)

hairsplitter
    n 1: a disputant who makes unreasonably fine distinctions

from The Collaborative International Dictionary of English v.0.48

Hairsplitter \Hair"split`ter\ (-spl[i^]t`t[~e]r), n.
   One who makes excessively fine or needless distinctions in
   reasoning; one who quibbles. "The caviling hairsplitter."
   --De Quincey.
   [1913 Webster]