Green’s Dictionary of Slang

1. a capital crime.

1835

Metropolitan Mag. 14 330: If they had me again, it should be for nothing less than a topper — no more lagging for me .

2. a hangman.

1938

J. Curtis They Drive by Night 275: All I got to do is weigh up when the topper’s likely to get to work and give a jump into the air. Well, down I come on sweet fanny adams and break my bleeding neck. It’s me who’s killed myself.

3. (UK/US prison) someone who has attempted suicide.

1943		J. Phelan Letters from the Big House 38: If only there was books, for young greens what might come in stir, so’s they’d know what to do. Be a sight less balmies and chokies and toppers, eh mates?
2000		Sun. Times News Rev. 12 Mar. 3: His roomies were the ‘cluckers’, ‘toppers’ (suicidal) and ‘fraggles.’.