Source : Webster's Revised Unabridged Dictionary (1913)

Forty-niner \For`ty-nin"er\, n.
   One of those who went to California in the rush for gold in
   1849; an argonaut. [Colloq., U. S.]

Source : WordNet®

forty-niner
     n : a miner who took part in the California gold rush in 1849