1. 坐起来;坐直
If you sit up, you move into a sitting position when you have been leaning back or lying down.

2. 使…坐起身来;让…直起身子
If you sit someone up, you move them into a sitting position when they have been leaning back or lying down.

3. 深夜未睡;熬夜
If you sit up, you do not go to bed although it is very late.

4. 突然关注;警觉
If something makes you sit up, it makes you suddenly pay attention to what is happening.

5. see also: sit-up

1. not go to bed

e.g. Don't stay up so late--you have to go to work tomorrow
We sat up all night to watch the election

Synonym: stay up

2. change to an upright sitting position

e.g. He sat up in bed