An sideline refers to the area around a playing field or court, but it can also mean something or someone that's secondary or not directly involved.
When we talk about sports, a sideline is usually an area near the action where coaches and players sit. But when we think about jobs or careers, a sideline is often a hobby or business that you do in addition to your main work. This means it's not your full-time job, but something you enjoy doing on the side. For example, a doctor might have a sideline as an artist, selling paintings online. Or someone who works at an office might make crafts and sell them on weekends.
How common is "sideline"?
Word sideline is considered uncommon in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
noun
A line at the side of something.
Something that is additional or extra or that exists around the edges or margins of a main item.
Example: She started the business as a sideline to her regular work and it ended up becoming the greater source of income.
A line for hobbling an animal by connecting the fore and the hind feet of the same side.
(sports) A line defining the side boundary of a playing field. Used in Canadian football, field lacrosse and basketball.
(sports, usually in the plural) The area outside the playing field beyond each sideline.
Example: The coach stood on the sidelines and bellowed commands at the team.
(figuratively) The outside or perimeter of any activity.
Example: to relegate to the sidelines
(Canada) A secondary road, especially a byroad at right angles to a main road.