It appears with this meaning in some dictionaries, for example, this one. In BrE, it still mainly has the meaning of not going to bed.
Student or Learner
I've heard 'wait up' in an American film used in a context similar to "Wait up. I'm also coming with you".
Although I have checked my dictionary, I have only found the meaning of 'not go to sleep at night because of waiting for somebody' which is not at all close to the one I've heard in the film. Can 'wait up' be used as a synonym of 'wait' with its core meaning?
Thank you very much!
"Wait up" means for someone to wait for you to catch up with them, to join them. It's not a replacement for a simple "wait" in all cases.