Green’s Dictionary of Slang

in a friendly manner.

1888		‘’Arry at a Radical Reception’ in Punch 12 May 219/2: She catted that chummy — oh law!
1915		S. Ford Shorty McCabe on the Job 19: He comes in smilin’ and chirky, though, [and] slaps me chummy on the shoulder.
1958		W. Motley Let No Man Write My Epitaph (1959) 110: They were talking very chummy-chummy.
1968		J. Hersey Algiers Motel Incident 117: You talk chummy-chummy to the bartender, and then you turn around and slap him on the back with a summons.