Suck in Definition 1 of 3Pro Tip 1/3Sudden Realization Used when someone suddenly understands something and reacts quickly. She sucked in her breath as the solution to the puzzle dawned on her. Pro Tip 2/3Shock or Surprise Shows someone is surprised or shocked by something unexpected. He sucked in a sharp breath when he saw the unexpected guest. Pro Tip 3/3Preparation Indicates preparation for an effort or challenge ahead. The athletes sucked in deep breaths before the race began.