On the side Definition 1 of 3Pro Tip 1/3Secondary Activity Implies a less important or occasional activity, not your main job or role. I'm a student, but I babysit on the side. Pro Tip 2/3Hidden Operations Suggests an activity might be secret or not openly acknowledged. He's an accountant, but on the side, he's a mystery novelist. Pro Tip 3/3Extra Income Often refers to a way to earn additional money beyond one's main job. She sells handmade jewelry on the side for extra cash.