sit up
verb
Definitions
To assume a sitting position from a position lying down.
Example: Despite being sick, Lorin sat up to greet the visitors.
To sit erect.
Example: Sit up straight, mister!
To show sudden interest or surprise.
Example: He sat up when we mentioned the increased pay package.
(of the ball) To bounce, especially to a comfortable height.
noun
Definitions
An exercise in which the abdominal muscles are contracted, bringing the torso of a person lying down to a more upright position.
Example: Susan's doing sit-ups to strengthen her muscles.