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.

“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.

To place on the sidelines; to bench or to keep someone out of play.

“The coach sidelined the player until he regained his strength.”

To remove or keep out of circulation or out of the focus.

“The illness sidelined him for weeks.”