sit up verb 7/ˈsɪt.ʌp/ noun 1forms 5 verb7 •To assume a sitting position from a position lying down.Despite being cancer-ridden, Lorin sat up to greet the visitors. •To sit erect.Sit up straight, mister! •To not go to bed (notionally remaining in a sitting position).I sat up all night waiting for her to come home. •To show sudden interest or surprise.He sat up when we mentioned the increased pay package. •To bounce, especially to a comfortable height.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 •To stop pedalling hard.Half way up the climb, Jones sat up and let Smith continue alone. •(India, historical) For a young lady to open her house for several successive nights for any visitors who wish to pay their respects. sit-up /ˈsɪt.ʌp/ noun1 •An exercise in which the abdominal muscles are contracted, bringing the torso of a person lying down to a more upright position. The degree of difficulty can be controlled by choosing from partial-range or full-range versions.Susan's doing sit-ups to strengthen her muscles. formssat up · sitting up · sits up · sit ups · sit-ups