The Collaborative International Dictionary of English v.0.48:

Derringer \Der"rin*ger\, n. [From the American inventor.]
   A kind of short-barreled pocket pistol, of very large
   caliber, often carrying a half-ounce ball.
   [1913 Webster]

WordNet (r) 3.0 (2006):

derringer
    n 1: a pocket pistol of large caliber with a short barrel