Meaning

• To assume a sitting position from a position lying down.

  "Despite being sick, Lorin sat up to greet the visitors."

• To sit erect.

  "Sit up straight, mister!"

• To show sudden interest or surprise.

  "He sat up when we mentioned the increased pay package."

• (of the ball) To bounce, especially to a comfortable height.

• To not go to bed (notionally remaining in a sitting position).

  "I sat up all night waiting for her to come home."

Meaning

• An exercise in which the abdominal muscles are contracted, bringing the torso of a person lying down to a more upright position.

  "Susan's doing sit-ups to strengthen her muscles."