side line

n. 1. (a) A line pertaining or attached to the side of a thing. (b) Specif., a line for hobbling an animal by connecting the fore and the hind feet of the same side. [ Webster 1913 Suppl. ]

2. A line of goods sold in addition to one's principal articles of trade; a course of business pursued aside from one's regular occupation. [ Webster 1913 Suppl. ]

3. A secondary road; esp., a byroad at right angles to a main road. [ Canada ] [ Webster 1913 Suppl. ]

4. (Sports) Either of two marked lines which delimit the side boundaries of a playing field, as in football or soccer. [ PJC ]

5. (Sports) [ plural ] The areas in a sports arena just beyond the sidelines{ 4 }, where coaches and alternative players sit or stand. [ PJC ]

on the sidelines not actively involved in an activity, such as a sporting event, but available for use when needed; in reserve; as, the coach kept him on the sidelines for a week. [ PJC ]