Because it amuses me:

Anyway, so now that I have sources for 1940s slang, I have to decide what to do with it. Figured I'd use it only sporadically for local color, like bleeped out curse words.

But I do think it could be funny, occasionally, to be utterly incomprehensible.

Like in a scene like this:

A generic old-fogey hero and his chirpy young sidekick end up having to get information from a bunch of street-tough wanna-be delinquents, basically an eye-witness statement.

"So before the snitch could snap a cap, the grifter squirted metal in him!"

The hero is completely lost: "I have no idea what you just said."

The sidekick pipes up helpfully, "Before the informant could shout, the conman shot him.
--

Another incident, they overhear someone admitting to a crime.

"So we've got the snooper stringin' like a sucker, but turns out he has plenty enough of swift..."

"Translation?" The hero asks, annoyed.

The sidekick grins. "They tried to swindle the sleuth and he got wise on 'em."

The hero glares at him.

"Sorry, couldn't resist."

---

And at some point, the sidekick should chirp: "Aw, the goon's gone and done a Gooseberry lay, now we'll never find him!"

The hero will give the kid a LOOK.

"Oh, all right. I'll stop now."
---

I think it's funny at least. :-)