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sit up

part of speech: phrase
definition 1: to sit so that one's back becomes straight and not bent over, or to move into a sitting position from a lying position.
You should sit up at your desk and try not to slouch.The patient sat up in bed and stretched out her arms to her husband.
definition 2: to cause (someone or something) to sit straighter or move to a sitting rather than lying position.
She sat her little boy up so she could give him his medicine.The child sat up all her dolls along the windowsill.
definition 3: to remain sitting and awake rather than going to bed.
We sat up late watching a movie.
definition 4: to suddenly take note of or be alarmed by something.
The students sat up when they heard that the final exam was being moved a day earlier.