sit up
• move from a lying or slouching to a sitting position.
• "Amy sat up and rubbed her eyes"
• refrain from going to bed until a later time than usual.
• "we sat up late to watch a horror film"
• suddenly start paying attention or have one's interest aroused.
• "perhaps boycotting a home match will make them sit up"
sit-up
noun
• a physical exercise designed to strengthen the abdominal muscles, in which a person sits up from a supine position without using the arms for leverage.
