Green’s Dictionary of Slang

1. don’t be silly, don’t tease.

1930		W.R. Burnett Iron Man 56: No fooling [...] Are you gonna help me out?
1953		J. Thompson Savage Night (1991) 81: No fooling. How did she happen to tell you about it?

2. a phr. meaning honestly, I’m not lieing; also as interrog.

1921		P. & T. Casey Gay-cat 108: He pussy-foots it right back where he belongs and no foolin’.
1926		R. Lardner ‘I Can’t Breathe’ in Coll. Short Stories (1941) 14: Anyway mother may as well get used to the idea because it is ‘No Foolin’’ this time and we have got our plans all made.
1938		C. Beaton Cecil Beaton’s N.Y. 99: No foolin’, you oughta hear what they say if they don’t like the show.
1947		S. Lewis Kingsblood Royal (2001) 151: No foolin’?
1966		(con. 1958) R. Farina Been Down So Long (1972) 47: Fat, hairless [...] sweat lines all over his belly — like a hogshead, no fooling.
1974		(con. late 1940s) E. Thompson Tattoo (1977) 454: See you, no foolin?