Sign on Definition 1 of 5Pro Tip 1/3Professional Commitment Used when officially accepting job offers or responsibilities. She was excited to sign on for the new project. Pro Tip 2/3Long-Term Engagement Implies a longer commitment than just agreeing casually. He signed on with the firm for a three-year term. Pro Tip 3/3Formal Agreement Involves formal contracts or agreements, not informal arrangements. When I signed on to the deal, I knew it was binding.