Green’s Dictionary of Slang

to suggest, to hypothesize.

1944		S.J. Perelman Crazy Like a Fox 115: Mind you, this ain’t dialogue— I’m just spitballing.
1952		N.Y. Herald Trib. 21 Sept. IV 1/2: You gather the gang and spitball until the wrinkles are ironed out.
1958		R. Chandler Playback 151: Just spitballing [...] I might say it could mean that he didn’t pay his bill.
1967		W.K. Zinsser Paradise Bit 148: I’m just spitballing, of course — just trying it on for size.
1975		L. Rosten Dear ‘Herm’ 122: I am not spit-balling about this, Leo, because I want to write badly.
1992		H. Erickson Baseball in the Movies 103: It’s possible, and I’m just spitballing here, that Prism Video was hoping [etc.].
2006		J. Finder Company Man 353: So this little girl figures she gets close to you — I’m just spitballing here — and maybe she finds something out.
2013		J. Stahl Happy Mutant Baby Pills 15: Maybe —I’m just spitballing here — maybe you make it more of a convenience thing.
2019		Twitter 29 Jan. 🌐 Or...just spitballing here...why don't we keep the unpleasant hatred of our closest neighbours in an attempt to appeal to racist cunts?
2021		T. Swerdlow Straight Dope [ebook] [Y]ou’re always good at hanging flesh on the bones of my ideas. That’s why it’s good when we spitball.