Sit in for Definition 1 of 2Pro Tip 1/3Temporary Substitute Indicates a short-term replacement for someone, not a permanent change. Tom will sit in for me just for today's presentation. Pro Tip 2/3Willingness to Help Shows willingness to assist by taking over someone's responsibilities. I can sit in for you while you recover from your surgery. Pro Tip 3/3Professional Context Often used in work environments to describe covering for a colleague. During the audit, Janet will sit in for our regular accountant.