The Collaborative International Dictionary of English v.0.48:

geezer \gee"zer\, n. [Dial. corrupt. of Guiser a mummer.]
   A queer old fellow; an old chap; sometimes, an old woman.
   [Contemptuous, Slang.]

   Syn: bloke.
        [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

geezer
    n 1: a man who is (usually) old and/or eccentric