Collins Dictionary defines wildest dreams as:
If you say that you could not imagine a particular thing in your wildest dreams, you are emphasizing that you think it is extremely strange or unlikely.
The idiomatic expression appears to be quite old as suggested by Etymonline
Wildest dreams attested from 1717.
and before that, "wild" seemed to be used figuratively mainly about different forms of excitements:
- Meaning "sexually dissolute, loose" is attested from mid-13c. Meaning "distracted with excitement or emotion, crazy" is from 1590s.
So, where does "wildest dreams" come from? What connotation of the term does the expression rely on to suggest something extremely unlikely? Was it originally an AmE idiomatic expression?