Learn English – What exactly is “Nuh-uh/Yuh-huh!”

'Wherein' is generally only used in formal writing. The Oxford English Dictionary gives the following senses and quotes. I have given the most recent quote, from which you can tell that the word is no longer generally used.

1. In what (thing, matter, respect, etc.)?

   1891 - Oh, Britannicus! wherein have we offended?

2. In which (place, material thing, writing, etc.)?

   1888 - Peering keenly into the shadow wherein she stood

3. In, at, during, or in the course of which (time).

   1819 - Length of days Wherein to act the deeds which are the stewards Of their revenue.

4. In which (matter, fact, action, condition, etc.); in respect of which.

   1889 - There began a round of pleasure for Julia wherein she was the central figure.

5. Into which.

   1585 - We came to an anker very nigh the castle, wherin when our patrone would followe vs [etc.].

For this one the etymology is useful.

It's a Latin word, formed with the prefix con- (meaning 'with') plus the verb descendere (meaning 'go down, descend'), and together meaning to lower oneself metaphorically in status by associating with inferiors.

This is Classical Roman culture, remember; it featured a rigidly enforced vertical social hierarchy, with rich nobles at the top, slaves at the bottom, and everybody else in between.

In Modern English, with a somewhat different social system — and extremely different social values — the word has come to mean not so much to treat everyone alike — which is now a good thing, after all — but rather to be perceived to be making an effort to treat everyone alike, especially one's presumed "inferiors".

Since this is a matter of interpretation, often by those who feel condescended to (note the Passive of an intransitive verb with a transitivizing preposition, like insist on), condescension is easy to complain about, but very hard to distinguish from personal dislike.