What do you call two words not belonging together in the same sentence?

There isn't a single, universally accepted term for two words that shouldn't be used together in the same sentence. However, depending on the context, you could use one of these terms:

Formal:

* Juxtaposition: This refers to the placement of two contrasting elements side-by-side. It's often used to highlight the difference between them.

* Incongruity: This emphasizes the lack of harmony or compatibility between the two words.

* Dissonance: This suggests a clash or lack of agreement between the words, creating an unsettling effect.

* Oxymoron: This is a specific figure of speech that uses two contradictory words together (e.g., "deafening silence"). While not exactly the same, it can apply to a situation where two words clash in meaning.

Informal:

* Clashing words: This is a simple and direct way to describe the situation.

* Odd couple: This term can be used if the two words are unexpectedly paired together.

* Mismatched words: This highlights the lack of fit between the words.

The best term to use will depend on the specific context and the desired level of formality.