The Collaborative International Dictionary of English v.0.48:

cosh \cosh\ n.
   a piece of leather-covered metal with a flexible handle; used
   for hitting people. [slang]

   Syn: blackjack, sap.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

cosh \cosh\ v.
   1. .
      [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

cosh \cosh\ v. t.
   to hit (someone) with a cosh or similar bludgeon, usually on
   the head. [slang]
   [WordNet 1.5 + PJC]

WordNet (r) 3.0 (2006):

cosh
    n 1: a piece of metal covered by leather with a flexible handle;
         used for hitting people [syn: blackjack, cosh, sap]
    v 1: hit with a cosh, usually on the head