Movies
Wonka! Butterscotch? Buttergin? You running something on the side here?
— Mr. Salt · Willy Wonka & the Chocolate Factory

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
A blustering accusation. Butterscotch sounds plausible for a sweet-maker, but "buttergin" hints at something boozy, so the speaker suspects a secret side business. The joke lives in the made-up word and the suspicious, demanding tone.
Context reviewed