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

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "geezer":
   Father Time, Methuselah, Nestor, Old Paar, centenarian, dotard,
   elder, gaffer, golden-ager, gramps, grandfather, grandsire,
   graybeard, nonagenarian, octogenarian, old chap, old codger,
   old dog, old duffer, old geezer, old gent, old gentleman, old man,
   old party, old-timer, older, oldster, pantaloon, patriarch,
   presbyter, senior citizen, septuagenarian, sexagenarian,
   the quiet-voiced elders, venerable sir, veteran