sit up phrasal verb ( see also ↑ sit )
1 . to be in a sitting position or get into a sitting position after you have been lying down:
He was sitting up in bed, reading his book.
She sat up and reached for her glass.
2 . sit somebody up to help someone to sit after they have been lying down
sit somebody up in/on etc
I’ll sit you up on the pillows and you’ll be nice and comfortable.
3 . to sit in a chair with your back straight:
Just sit up straight and stop slouching.
4 . to stay up very late:
Sometimes we just sit up and watch videos all night.
5 . sit up (and take notice) to suddenly start paying attention to someone, because they have done something surprising or impressive:
If Maria succeeded, then everyone would sit up and take notice.