Green’s Dictionary of Slang

the bar at a golf club; esp. used by golfers but understood more widely.

1901		W.G. van T. Sutphen [title] The nineteenth hole, being tales of the fair green .
1913		T.A. Dorgan Indoor Sports 5 Dec. [synd. cartoon] This nineteenth hole is too popular.
1920		Ade Hand-made Fables 11: The Match Play was a pleasant Relief, because Golf will make you forget everything except the 19th Hole.
1948		Time 19 Jan. 52/3: He dug into his pocket for $10,000 in prize money, played more for the fun of it than in the hope of beating anybody, and helped entertain on the 19th hole [DA].
1969		R.L. Orosa What’s in a (Nick)name?: and Other Light Essays 41: After that game, the divot diggers continued their conversation for three hours at the nineteenth hole (the clubhouse).
1977		P. Schlafly Power of the Positive Woman 7: If I were stymied by a slice in my golf drive, I would seek lessons from a pro rather than join the postmortems in the bar at the ‘nineteenth hole’.
2002		B. Dekoven Well-Played Game 102: The nineteenth hole is, as a matter of fact, not on the course at all. [...] The nineteenth hole is a bar where we can soften the memory of the game with beer.