What is the meaning of Sit_up?

To assume a sitting position from a position lying down.

To sit erect.

To show sudden interest or surprise.

To bounce, especially to a comfortable height.

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

For a young lady to open her house for several successive nights for any visitors who wish to pay their respects.

Alternative spelling of sit-up

Source: wiktionary.org