“What I had in mind was boxing the compass”

....and getting to have sex with a gorgeous(if mature) woman played by Felicia Farr, who was Jack Lemmon's wife at the time and on to his death in 2001.

Now, THAT's an in joke.

Younger viewers can't seem to believe Walter Matthau as managing to bed such a gorgeous woman so quickly, but look to the plot:

He's a daring guy going up against the mob. She respects him.
He's likely to get killed the next day. She pities/honors him. Its like a "last supper."
She's a secretary to a Reno mob banker ...possibly some prostitution in her younger days, over there at the Chicken Ranch we see in the movie(the Mustang Ranch in real life.)
Her "current boyfriend" is that Reno mob banker(John Vernon) and he's not much better looking than Matthau.
Walter Matthau in these years was as good looking as he would ever get and managed to play some romantic roles ...polling showed that women liked him for his laid back manly manner.