Jack is sick today and doesn't really seem with it.
More Examples
Used in the Middle of Sentence
That cutter is no good to me now. I could have done with it yesterday.
Used in the Ending of Sentence
We had no choice but to grit our teeth and get on with it.
It was marked as seconds, but I can't find anything wrong with it.
Jack is sick today and doesn't really seem with it.
Meaning of with it for the defined word.
Grammatically, this idiom "with it" is a phrase, more specifically, a prepositional phrase. It's also a preposition, more specifically, a prepositional phrase.