There are some nice answers here, but having thought about it for a bit (I am English) I'm not at all sure that it is really appropriate to talk about the "opposite" of "under the weather".

"Under the weather", as has been mentioned, means "slighlty ill" and implies a little bit miserable or depressed as a result. It is exactly the kind of expression you would use about someone who has a cold, or mild flu (definitely not Covid-19).

There are multiple expressions for someone feeling "OK", not ill, not suffering from a cold, etc. But are these really opposite to a very specific degree of illness and slight depression or sluggishness?

Is there a word for the "absence of blue"?

There are some nice answers here, but having thought about it for a bit (I am English) I'm not at all sure that it is really appropriate to talk about the "opposite" of "under the weather".

"Under the weather", as has been mentioned, means "slightly ill" and implies a little bit miserable or depressed as a result. It is exactly the kind of expression you would use about someone who has a cold, or mild flu (definitely not Covid-19).

There are multiple expressions for someone feeling "OK", not ill, not suffering from a cold, etc. But are these really opposite to a very specific degree of illness and slight depression or sluggishness?

Is there a word for the "absence of blue"?