1
general::
Also, get with it. Be or become knowledgeable about the current or latest trends, fashions, or ideas, as in She just turned 60, but she's still very much with it, or Get with it, Dad, that kind of razor hasn't been made for years. [Slang; 1920s]
American Heritage Idioms
2
general::
Phrase(s): with it
1. Inf. alert and knowledgeable. • Jane isn’t making any sense. She’s not really with it tonight. • Jean’s mother is not really with it anymore. She’s going senile. • Peter’s not with it yet. He’s only just come round from the anesthetic. 2. Inf. up-to-date. • My parents are so old-fashioned. I’m sure they were never with it. • Why do you wear those baggy old clothes? Why aren’t you with it?
McGrawhill's American Idioms And Phrasal Verbs