Green’s Dictionary of Slang

ripe for duping; thus naut. jargon sucking Nelson, a midshipman.

1834		Marryat Peter Simple (1911) 22: What a nice young gentleman you have there. He looks like a sucking Nelson. I say, my pretty young officer, could you lend me a shilling?
1849		C. Kingsley Alton Locke (1850) 55: Hullo, young ’un [...] Oh! eh! Forgot the latch-key you sucking Don Juan, that’s it, is it?