The gentleman's "Fuck you motherfucker"

A quick and curt way to end a conversation, putting a thin polite spin on the rude abruptness.

Most uses of this phrase are referencing actor Gene Wilder's portrayal of Willy Wonka in the 1971 movie "Willy Wonka & the Chocolate Factory," where Wonka informs Charlie he has lost the contest because he drank the burp soda, a breach in the contract he signed. "You lose! Good day, SIR!" Wonka screams at Charlie before turning back to his desk.

Linking to an animated gif of the scene with the audio intact is a popular way to assert you are done with an argument on the internet.

One day, two marijuana enthusiasts smoked some rather potent weed. This weed did things to them like nothing they had ever smoked before. At the end of the night one of the stoners dropped off the second stoner, but before driving away he shouted "Good Day Sir" and drove off without saying another word. Ever since that day stoners occasionally use this expression in place of saying "goodbye"

these three words may seem innocent, but they can trigger people almost instantly.
you may be saying " good day sir", but what they hear is "you're gay sir".
"good day sir" also goes hand in hand with "you're gay sir"