Can anyone tell me what is the meaining of look who's up
Hard to tell with no context, Guest.

'Look who's up' could mean 'notice who is batting' (baseball) or 'notice who is awake and out of bed', or 'notice whose turn is next' (other sports, competitions, etc.). And there may be other interpretations-- can you post the context?
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here is the dialogue.
A: Good morning everyone.
B: Well, look who' up?

"Notice who is out of bed" can make sense

That's it. The second speaker is a bit surprised to see the first speaker awake so early.

Basically , can't sleep ,bored, and honestly was really curious , who was going to respond.

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