Green’s Dictionary of Slang

ballgame n.

[SE ballgame, a sporting event, esp. a baseball game]

(orig. US) a state of affairs, a situation.

[US]M. Bodenheim Naked on Roller Skates 127: Ain’ no double plays coming off in this ballgame, chicken.

In phrases

end of the ballgame (n.) (US)

the definitive conclusion to something.

V. Bugliosi Outrage: The Five Reasons Why O.J. Simpson Got Away With Murder [Internet] If your blood is found at the murder scene, as Simpson’s was conclusively proved to be by DNA tests, that’s really the end of the ball game.
that’s the ballgame (also there goes the ballgame)

(US) a phr. meaning that’s it, no arguments accepted, forget it.

[US]J. Steinbeck Wayward Bus 164: There goes your ballgame.
[US]L. Uris Battle Cry (1964) 152: O.K., sister, that’s the ball game.
[US]H.S. Thompson letter 19 Oct. in Proud Highway (1997) 290: That’s about the ballgame, McGarr.
[US]S. Greenlee Spook who Sat by the Door (1972) 6: A small shift and there goes the ball game.
[US]T. Thackrey Thief 7: Around the corner ... and that’s the ball-game!
[US]C. Hiaasen Double Whammy (1990) 269: As soon as they spot those two guys, that’s the ball game.
[US]G. Indiana Rent Boy 106: Guy’s got a knife, you might have a chance, but a gun, man, one bang and that’s the ballgame.
[US]Kevin Devine ‘Ballgame’ [lyrics] Yeah when you realize it’s a pattern and not a phase, it’s what you’ve become and it’s what you will stay... that’s the ballgame.
whole new ballgame (n.) (also different ballgame)

(orig. US) a radically new situation (to which one will be forced to adapt).

[US]N.Y. Times 14 Jan. 28: The first fish in constitutes a record and it’s a whole new ball game.
[US]Realist Sept. 23/2: The statement was all ready when up stepped Sirhan Sirhan, and in ten seconds he made it a whole new ball game.
[US]E. Tidyman Shaft 31: Seeing him and going up against him is a new ball game.
[US]C. McFadden Serial 86: ‘A viable alternative to the family unit’ was what Harvey called ‘a whole new ball game.’.
[UK]P. Theroux Picture Palace 142: It’s a different ball game altogether.
[Aus]M. Bail Homesickness (1999) 107: What our American friends, I think, call a ‘whole new ball game’. Dreadful expression.
[US]D. Burke Street Talk 2 174: ‘I’m glad we’ve finally agreed on which movie we’re going to see. Now suppose we invite Bob?’ ‘Wait! That’s a whole new ball game!’.
[UK]Guardian G2 6 Dec. 10: Night driving is a different ball game to cruising around in the day.
[US]Source Aug. 32: You ain’t a lawyer no more, Dave; you a gangsta now. You on the other side. Whole new ballgame.