My idea is a class about the origin, change, and regional dialects of the English language.
Here are ten words that, at least in terms of one sense, have been demoted by an association with rural dialect.
These are terms or phrases used in certain situations that have a second, implied meaning which is usually a bit difficult to guess.
His face contorted with thought.
If no one uses the word it is not a colloquialism, it is simply obsolete.
Colloquialisms are a bit more formal than slang.