[a reporter mumbles an unintelligible insult at Mac as he exits the office]
MacWade: What was that?
Reporter: I said you were a... uh... I said you had dirty plaster.
Kind of surprising that the Hays Office allowed them to slip that in (so to speak).
Essentially the same gag is used in Swing Time (1936).
Fred Astaire and Victor Moore's characters have a run-in with a police officer, after which Moore says something that is obscured by street noise.
When the officer asks what he said, he points to nearby street construction, and says "I said 'Look out for the great big ditch.'"
Yes, screenwriters sure did like to slip things in around the prohibitions. My favorite so far is from the original "Star Trek" TV series, about 50 years ago. McCoy to Spock: "Are you out of your Vulcan mind?"
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I never thought about "Vulcan" being one of those slightly-changed-pronunciation euphemisms. Now I can't un-hear it!
But then, my Star Trek viewing is forever affected by the fact that I originally watched the show in its first run, when I was just an innocent little girl who would never have noticed such things.