Definition of "sideline"

Like most other fishes, the dragonet varies slightly in colour in different individuals, and at different seasons of the year. Mr. Pennant describes the pupils of the eyes to be of a rich sapphirine blue; the irides fine fiery carbuncle; the pectoral fins light brown; the side-line straight; the colours of the fish yellow, blue, and white, making a beautiful appearance when fresh taken.

1809, William Nicholson, “CALLIONYMUS”, in The British Encyclopedia, or Dictionary of Arts and Sciences; […], volume II (B … E), London: Printed by C[harles] Whittingham, […]; for Longman, Hurst, Rees, and Orme, […], column 1

Conservative Catholics, Relegated to Sidelines, Denounce Papal Gathering [title]

2023 October 3, Jason Horowitz, “Conservative Catholics, Relegated to Sidelines, Denounce Papal Gathering”, in The New York Times‎

Gomez got his chance with Gary Cahill sidelined and took it superbly with an assured performance as part of a three-man defence Southgate now looks certain to use at the World Cup.

2017 November 14, Phil McNulty, “England 0-0 Brazil”, in BBC News‎

Subplots that might have been fun to explore were relegated or eventually sidelined altogether in the case of characters like Gendry, who disappeared for years and finally resurfaces as a blacksmith in King’s Landing, literally waiting for the call to his hero’s journey.

2017 August 13, Brandon Nowalk, “Oldtown offers one last game-changing secret as Game Of Thrones goes behind enemy lines (newbies)”, in The Onion AV Club‎

During 1971-72, spare Type 4s on other regions enabled the whole class to be sidelined, with Class 43s going first.

2022 January 12, David Clough, “From Germany with love: a Warship retrospective”, in RAIL, number 948, page 49

Prigozhin’s exclusion from Ukraine would sideline one of Russia’s most recognisable war figures.

2023 June 14, Pjotr Sauer, Andrew Roth, “Putin sides with military chiefs over placing Wagner under direct control”, in The Guardian‎