Definition of "nark"

So I went and laid down on the grass. While laying there I piped a reeler whom I knew. He had a nark (a policeman's spy) with him. So I went and looked about for my two pals, and told them to look out for F. and his nark.

1879 October, J[ohn] W[illiam] Horsley, “Autobiography of a Thief in Thieves’ Language”, in Macmillan’s Magazine, volume XL, number 240, London: Macmillan and Co. […], page 505, column 1

It’s a—well, it’s a copper’s nark, as you might say. What else would you call it? A sort of informer.

1912, George Bernard Shaw, “Act I”, in Pygmalion‎

When we got to the Chief of Police's office a crowd of the most dreadful-looking scoundrels, obviously police narks, informers, and spies of every kind, were hanging about outside the door.

1938 April, George Orwell [pseudonym; Eric Arthur Blair], chapter XII, in Homage to Catalonia, London: Secker & Warburg