The Collaborative International Dictionary of English v.0.48:

Sneaker \Sneak"er\ (sn[=e]k"[~e]r), n.
   1. One who sneaks. --Lamb.
      [1913 Webster]

   2. A vessel of drink. [Prov. Eng.]
      [1913 Webster]

            A sneaker of five gallons.            --Spectator.
      [1913 Webster]

   3. A type of soft shoe with a flat, pliable, typically rubber
      or other soft sole, and canvas-like upper, used in sports
      such as tennis, or for comfort. Called sneaker because
      they give no warning of one's approach. Usually used in
      the pl.
      [Webster 1913 Suppl. +PJC]

   4. A punch bowl. [Obs.] --Spectator.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

sneaker
    n 1: a canvas shoe with a pliable rubber sole [syn: gym shoe,
         sneaker, tennis shoe]
    2: someone acting as an informer or decoy for the police [syn:
       fink, snitch, snitcher, stoolpigeon, stool pigeon,
       stoolie, sneak, sneaker, canary]

The Jargon File (version 4.4.7, 29 Dec 2003):

sneaker
 n.

    An individual hired to break into places in order to test their security;
    analogous to tiger team. Compare samurai.