sit up {v.} 1. To move into a sitting position. * /Joe sat up when he heard the knock on his bedroom door./ 2. To stay awake instead of going to bed. * /Mrs. Jones will sit up until both of her daughters get home from the dance./ * /We sat up until two A.M. hoping for news from Alaska./ 3. {informal} To be surprised. * /Janice really sat up when I told her the gossip about Tom./