Green’s Dictionary of Slang

to inform against, to betray; to become an informer.

1857		Times 5 Dec. 12/5: I’ll have some more money, or else I’ll ‘round’ on you.
1866		Wild Boys of London I 23/2: Pug ain’t a pal o’ mine, but I don’t round on him.
1877		Hull Packet 6 Apr. 5/2: Don’t ‘round on a pal’.
1884		Dundee Courier 13 June 7/5: You knows what I’m hidin’ for, Jerry? [...] but you’ll not round on me, wil you?
1885		M. Davitt Leaves from a Prison Diary I 22: They [i.e. senior criminals] never ‘round’ upon each other, while they hold all ‘coppers’ (prison informers) in detestation.
1890		Bulletin (Sydney) 2 Aug. 10/1: O Blessed Interpreter! / Scene – Suburban Police-court / Witness: ‘Prisoner said he’d plug me if I blew th’ gaff, yer Wusshup.’ / Stipendiary: ‘What does that mean?’ / [...] Witness: ‘Well, rounded on ’im, yer Wusshup.’.
1904		Marvel 12 Nov. 8: He is quite willing to round on his old associates.