Definition of "sit up"

sit up

verb
third-person singular simple present sits up, present participle sitting up, simple past and past participle sat up
1. (intransitive) To assume a sitting position from a position lying down.
  Quotations
  She was awakened by a shock, so sudden and severe that if Dorothy had not been lying on the soft bed she might have been hurt. As it was, the jar made her catch her breath and wonder what had happened; and Toto put his cold little nose into her face and whined dismally. Dorothy sat up and noticed that the house was not moving; nor was it dark, for the bright sunshine came in at the window, flooding the little room. She sprang from her bed and with Toto at her heels ran and opened the door.
  1900 May 17, L[yman] Frank Baum, The Wonderful Wizard of Oz, Chicago, Ill., New York, N.Y.: Geo[rge] M. Hill Co.
2. (intransitive) To sit erect.
3. (intransitive) To show sudden interest or surprise.
4. (chiefly sports, of the ball) To bounce, especially to a comfortable height.
  Quotations
  And within five minutes of the change United were level. Gabriel Obertan tuned his marker inside out and delivered a cross from the right that hit Hernandez and sat up perfectly for Owen to nod home.
  2011 January 29, Ian Hughes, “Southampton 1 - 2 Man Utd”, in BBC‎
5. To not go to bed (notionally remaining in a sitting position).
6. (India, historical) For a young lady to open her house for several successive nights for any visitors who wish to pay their respects.
noun
plural sit ups
1. Alternative spelling of sit-up