from
The Collaborative International Dictionary of English v.0.48
Sideline \Sideline\, Side line \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]