Merriam-Webster's Collegiate® Dictionary, 11th Edition
SIDELINE
I. noun
Date: circa 1862
a line at right angles to a goal line or end line and marking a side of a court or field of play for athletic games,
2. a line of goods sold in addition to one's principal line, a business or activity pursued in addition to one's regular occupation,
3. the space immediately outside the lines along either side of an athletic field or court, a sphere of little or no participation or activity,
II. transitive verb
Date: 1943
to put out of action ; put on the sidelines