Wait up Definition 1 of 2Pro Tip 1/3Nightly Wait Used when someone delays going to sleep to see or support another person. Mom promised to wait up until I finished studying for my exam. Pro Tip 2/3Caring Gesture Implies concern for someone's well-being, showing you'll stay awake for them. Don't rush. I'll wait up for you, drive safely. Pro Tip 3/3Sincere Assurance Offers comfort, suggesting that someone will not be alone upon return. After her long trip, her husband said he'd wait up.