Green’s Dictionary of Slang

to hit with a cosh or blackjack.

1856		Argus (Melbourne) 1 Dec. 6/2: ‘Mr. Price thinks no more of knocking a man down than looking at him. I know him to give orders to Sergeant Smith to neddy a man, and he refused to do it. Mr Price replied, “If you do not neddy that man I shall discharge you.” Serjeant Smith then neddied immediately’.
1865		Leaves from Diary of Celebrated Burglar 40/2: The fellow must be ‘neddied,’ no matter who he is.
1874		M. Clarke Term of His Natural Life (1897) 213: They broke locks, and burst doors, and ‘neddied’ constables.