The number "Good Afternoon" in the recent musical Spirited is mining a vein of comedy peculiar to adaptations of A Christmas Carol, the Charles Dickens novel first published in 1843. Let's keep in mind Spirited isn't trying for any historical authenticity: rather, it's set in our times (though with wacky time travel) and is self-consciously riffing on previous adaptations. So this is really more of a "know your meme" type of issue.

It's pretty clear where the idea of an insulting "good afternoon" in an adaptation of A Christmas Carol comes from. In a passage early in the novel itself, Scrooge repeatedly "good afternoons" his kind-hearted nephew Fred who has come by to invite Scrooge to visit for Christmas and inform Scrooge of his recent marriage:

"Why did you get married?" said Scrooge.

"Because I fell in love."

"Because you fell in love!" growled Scrooge, as if that were the only one thing in the world more ridiculous than a merry Christmas. “Good afternoon!”

“Nay, uncle, but you never came to see me before that happened. Why give it as a reason for not coming now?”

“Good afternoon,” said Scrooge.

“I want nothing from you; I ask nothing of you; why cannot we be friends?”

“Good afternoon,” said Scrooge.

“I am sorry, with all my heart, to find you so resolute. We have never had any quarrel, to which I have been a party. But I have made the trial in homage to Christmas, and I’ll keep my Christmas humour to the last. So A Merry Christmas, uncle!”

“Good afternoon!” said Scrooge.

“And A Happy New Year!”

“Good afternoon!” said Scrooge.

His nephew left the room without an angry word, notwithstanding.

It should be obvious but perhaps bears exposition that "good afternoon" is a dismissal not a greeting: you say it at the end of the interaction to signpost that the interaction has ended. And it is quite dismissive in this instance! The narrator is basically telling you Fred should have been angry but wasn't.

Later in the novel, Scrooge curtly disposes of men soliciting donations with a "Good afternoon!"

Anyway, these scenes are beloved by people writing adaptations. Scrooge rudely shouts "Good afternoon!" to get rid of people who are too Christmasy for him and they react with astonishment to his rudeness. And as Will Ferrel explained, older movie adaptations "use that vernacular as if it were a bad word," so for Spirited he and Ryan Reynolds built a whole number around it.

Basically, the number is based on a jokey idea that you could simply come up to someone and say, "Good afternoon!" as some kind of all-purpose insult. But that's a joke about the tradition of adaptations--the Scrooge character lustily bellowing "Good afternoon" at normie jingle-janglers who are outraged by his cheek--not the actual mid-19th century.

In reality, tapping someone on the shoulder and saying "Good afternoon" would not be so much insulting as confusing. Imagine coming up to someone and saying, "Bye-bye." That would be weird. You would seem like a weirdo. Now of course, if there was an ongoing interaction and you cut it off, then you'd seem like a jerk, which of course is exactly what Scrooge is going for in the novel (and what the adaptations play up).