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.

[TahlilGaran] Dictionary of Contemporary English ▲

ˈsit-up British English, sit‧up American English /ˈsɪtʌp/ noun [countable]
an exercise to make your stomach muscles strong, in which you sit up from a lying position, while keeping your feet on the floor ⇒ crunch:
Jerry says he does two hundred sit-ups a day.

[TahlilGaran] Dictionary of Contemporary English ▲

sit up
to stay awake past the time that you usually go to sleep.
We sat up talking half the night.

Many nights I sat up until dawn, trying to get my term papers finished on time.

[TahlilGaran] English Idioms Dictionary ▲

sit up(1)
v.
1. To move into a sitting position.
Joe sat up when he heard the knock on his bedroom door.
2. To stay awake instead of going to bed.
Mrs. Jones will sit up until both of her daughters get home from the dance.
We sat up until two A.M. hoping for news from Alaska.
3. informal To be surprised.
Janice really sat up when I told her the gossip about Tom.

[TahlilGaran] English Idioms Dictionary ▲

sit-up(2)
n. A vigorous exercise in which the abdominal muscles are strengthened by locking one's feet in a fastening device and sitting up numerous times.
Do a few sit-ups if you want to reduce your waist.

[TahlilGaran] English Idioms Dictionary ▲