From Longman Dictionary of Contemporary Englishsit up phrasal verb1 SITto 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 sit somebody up to help someone to sit after they have been lying downsit somebody up in/on etc I’ll sit you up on the pillows and you’ll be nice and comfortable.3 SITto sit in a chair with your back straight Just sit up straight and stop slouching.4 SLEEPto stay up very late Sometimes we just sit up and watch videos all night.5 sit up (and take notice)ATTENTION 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. → sit→ See Verb table
Examples from the Corpussit up• When I got home, Nigel was sitting up in bed.• He sat up in his chair when I started talking about Chris.• Sit up like a big girl, and eat your dinner.• Sit up straight and put a cushion behind your lower back.• Cadets here are taught to always dress neatly and to sit up straight.sit in/on etc• She sat herself up in bed and swung her little legs over the side.• Olivia could do nothing but sit perched up on her chair.• Mr Grange sits bundled up in his aluminum chair and cuts snowflakes out of lined notebook paper.• He just sat there up on his haunches, staring at it between her fingers.• Edwin would sit, propped up in his room, with plans of the estates and charts of the crops.• Jeanne sat hunched up on the wooden cot which served as a bed.sit up straight• He sat up straighter and locked both hands on the wheel.• Children were to sit up straight and put their chins to their chests.• They all sat up straight as Patrick walked among them.• They sit up straight, eyes forward, and concentrate on their teacher.• Sit up straight in your chair and pay attention.• They never put their elbows on the table, and they sat up straight instead of slouching.• He sits up straight to do up the buttons or velcro fastenings.• Léonie sat up straight, tucked her feet to one side, put her hands round her knees.• She sat up straight with folded hands, frowning at me.sit up (and take notice)• I was still groggy, but I could sit up.• He sat up and stared at the sky in wonder.• After a bit they sat up and watched the welcome breeze work like an animal through the silver-green barley.• They sat up side by side in the bed, naked, listening, but Valerie no longer felt safe.• Léonie sat up straight, tucked her feet to one side, put her hands round her knees.• Carol was dying, and he cried out in his sleep and sat up trembling with cold sweats in the heat.• I sat up, wondering what the hell!sit-upˈsit-up British English, situp American English /ˈsɪtʌp/ noun [countable] DSOan 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.
Examples from the Corpussit-up• Toe touches and sit-ups should be avoided; they are particularly harmful to anyone who has osteoporosis in the spine.• Then he did exercises he had learned in the Armystraddle jumps, deep knee bends, sit-ups and push-up.• He also did sit-ups, but not in front of me.• An acute attack of dizziness while doing my sit-ups.• The sit-ups start with the back off the board, and end before the upper body reaches the vertical position.• Before you think it's too late to do those sit-ups, though, think again.• I could get away with only forty twist sit-ups.