Language is linear. Use and context establish tone, with an expectation for its continuation. When one suddenly uses a register, dialect, or vocabulary at a significant distance from that previously employed, the effect is really freakin' weird.
There's a certain humor in playing with different levels of language use, and the common trick is to mix "sophisticated" language (such as Spock Speak, Antiquated Linguistics, Sesquipedalian Loquaciousness, Gratuitous Foreign Language, or extremely formal Received Pronunciation British) with "unsophisticated" language (such as the Cluster F-Bomb, Totally Radical, or Buffy Speak), with the necessary awkwardness on both sides. Common examples include:
A quote misattribution ("In the words of the great Oscar Wilde, STFU n00b").
Suggesting a "technical", "professional", or obscure foreign term, followed by slang or profanity ("Your engine is what we in the business describe as 'completely screwed'." "He's what Freud used to call 'spooky'." "As the French say, you, my friend, are le utter cock.") or following a lengthy formal or descriptive analysis.
Slang speech or vulgarity is quoted in an official capacity or environment ("Following the officer's formal warning, the accused threatened to 'pop a cap' in the officer's posterior").
Many varieties of Flowery Insults, especially when used in a diplomatic or government context.
Slang delivered innocuously in a formal speech, especially from someone upper-class.
An attempt at Jive Turkey slang couched in academic or formal terminology, often drifting into Totally Radical.
A normally formal character resorting to profanity due to intense circumstances (see: Precision F-Strike).
A subtrope of Bathos.
Compare Buffy Speak, Jive Turkey, Delusions of Eloquence, Bread, Eggs, Milk, Squick, Technical Euphemism. Textbook Humor is often of this type. Not to be mistaken by name for Wicked Cultured. Precision F-Strike is a subtrope. With Due Respect is a common way of getting to this trope. See also Foreign Cuss Word.
Contrast with Expospeak Gag, where a slangy phrase is disguised in excessively formal language (although they can overlap if the speaker then "clarifies" what they were saying, probably while raising an eyebrow).
