Slang on the other hand, can be used by a limited group of people who
use incorrect words to refer to a thing, but anyone outside group will
not understand.

An idiom is a word or phrase that is not interpreted literally. Slang is a word or phrase that belongs to a limited group of speakers or context.

Slang does not refer to  "incorrect" words, and it's often widely understood outside its context. Within a slang expression's limited group or context, the slang follows its own rules and is "correct."

For example, woke is a modern slang word meaning that the subject is highly aware of social injustices. It is generally used by the young, and when older generations try to use it, there is a feeling that they're not part of the group that uses that word; it's awkward. It's meant to be interpreted metaphorically: someone who is woke is awakened to social injustices.

Under the weather is a common idiom whose meaning has nothing to do with the weather; it means the subject is sick. It can be used in all but the most formal of contexts and groups.

-- 
SuStel
http://trimboli.name