Green’s Dictionary of Slang

1. (UK Und.) a man who acts as a lookout man or a diversion for a dropper n.¹

1941		V. Davis Phenomena in Crime 48: The ‘topper’ [...] plays a double role in the scheme of things. Firstly, as a look-out man for the cheque stealers during their early-morning letter-box raids, and secondly, ‘minds’ the ‘dropper’ while the latter is negotiating a forged cheque in a bank. The ‘topper’ causes a diversion in the bank to draw attention from the ‘dropper’ in the event of the counterfeit being discovered as such. The latter makes his get-away in the hullabaloo.
1956		‘Charles Raven’ Und. Nights 81: Dropping is dodgy work, so dodgy that the penman, who never drops himself, has to send a minder, known as a topper, to keep an eye on the dropper.

2. (Aus./N.Z.) an informer.

1953		I. Hamilton Till Human Voices Wake Us 99: When you’re in jail your sympathy is with the crimmos, unless you’re just a natural topper.
1973		‘John Justin’ Prisoner 23: Do you know what a topper is? [...] He tells the screws everything he hears. Hopes he’ll get extra remission.
1982		G. Newbold Big Huey 255: topper (n) Informer.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Top off. 2. To betray or inform upon. Thus a topper is an informer.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 192/1: topper n. an informer.