Green’s Dictionary of Slang

(orig. US) in a friendly manner.

1958		(con. 1950) E. Frankel Band of Brothers 3: When the brass starts actin’ buddy-buddy [...] stand by for a ram.
1967		M. Braly On the Yard (2002) 343: The question [...] reminded Red of the interrogatory technique of the kind of detective who came on buddy-buddy.
1971		R.D. Pharr S.R.O. (1998) 354: ‘It’s colder than a whore’s pussy out there this morning,’ Lorraine exlcaimed, all buddy-buddy.
1999		Guardian Guide 2–8 Oct. 15: There’s no way I’m going to hang out, going buddy-buddy with a bunch of racist skinheads.