Green’s Dictionary of Slang

a street thief who robs drunks under the pretence of helping them up from the gutter into which they have fallen (or been pushed).

1887		Cincinnati Enquirer (OH) 24 Dec. 12/2: They gave Chandler the reputation of being a ‘stander-up’ of drunken men, or one who makes a business of robbing those who fall asleep [...] under somnolent effects of liquor.
1890–1904		Farmer & Henley Sl. and Its Analogues.