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1. to rise from a lying to a sitting position. When the alarm went off, he sat up and put his feet on the floor. She couldn't sleep, so she sat up and read a book.
2. to sit more straight in one's seat; to hold one's posture more upright while seated. Please sit up. Don't slouch! You wouldn't get backaches if you would sit up.
1. Rise to a sitting position from lying down, as in The sick child sat up and asked for a drink of water. [Early 1200s]
2. Stay up later than usual, as in The nurse sat up with her all night long. [Mid-1500s]
3. Sit with the spine erect, as in She was always telling the students to sit up. [Early 1700s]
4. Become suddenly alert, as in The students sat up when he brought up the test. The same sense appears in the related sit up and take notice, as in When he mentioned the arrival of a movie star, they all sat up and took notice. [Late 1800s]
sit up (and take notice)suddenly start paying attention or have your interest aroused. informal
1. To rise from lying down to a sitting position: The patient sat up for her meal.
2. To sit with the spine erect: The nanny told the children to sit up.
3. To stay up later than the customary bedtime: My parents sat up waiting for me to come home.
4. To become suddenly alert: The students sat up when the teacher mentioned the test.