Green’s Dictionary of Slang

1. a sports fan, esp. a baseball fan.

1889		World (N.Y.) 19 Oct. 2/3: A first-class rooter roots with his hands and his feet and not with his mouth.
1899		Ade Fables in Sl. (1902) 27: Once upon a Time a Base Ball Fan lay on his Death-Bed. He had been a Rooter from the days of Underhand Pitching.
1904		Sydney Sportsman (Surry Hills, NSW) 7 Sept. 6/5: The Jessop rooters ywelled, ‘Foul’.
1908		N.Y. Herald in Fleming Unforgettable Season (1981) 46: ‘Larry’ Fassett, once owner of the Albany team and a thirty-third-degree Giant ‘rooter.’.
1912		Van Loan ‘McCluskey’s Prodigal’ in Ten-Thousand-Dollar Arm 255: The home rooters sat back [...] and ceased their gloomy predictions of defeat.
1924		P. Marks Plastic Age 75: I ’ve seen a lot of football games, and I’ve seen lots of rooters.
1937		A.E. Duckett ‘Truckin ’round Brooklyn’ in N.Y. Age 30 Jan. 7/1: [of basketball fans] [P]lenty of hot discussion from rooters of both sides.
1943		in G. Legman Limerick (1953) 293: A young baseball-fan named Miss Glend / Was the home-team’s best rooter and friend.
1951		Star (Marion, OH) 17 Nov. 2/5: Notre Dame was taking a real shellacking from Michigan State last Saturday and an Irish rooter in the stands was dying hard.
1959		E. De Roo Young Wolves 140: The noisy rooters in the front row across the floor started up their vexing ‘Ya-hoo, ya-hoo, West is through!’ cheer.
1972		D. Jenkins Semi-Tough 197: He was sort of a sentimental Jet rooter.
1983		S. King Christine 179: A second bus, loaded up with cheerleaders, the band, and all the LHS kids who had signed up as ‘rooters.’.

2. an enthusiast, a supporter, in non-sports context.

1906		E. Hubbard Love, Life and Work 🌐 Doctor Chapman and his professional rooters may roll in cheap honors, be immune from all useful labor and wax fat on the pay of those who work.
1912		Bulletin (Sydney) 22 Aug. 11/3: For the first time in convention-history, at the night sessions at Baltimore, many scores of the delegates and alternates came in evening clothes [...], nor did even one of the 12,000 ‘rooters’ in the galleries make disrespectful remarks about it.
1920		S. Lewis Main Street (1921) 416: The citizens accompanied it as ‘rooters,’ in a special car.
1929		E. Dahlberg Bottom Dogs 66: Mush Tate started to megaphone thru his hands and got some rooters for the newcumber.
1952		W. Winchell On Broadway 12 Aug. [synd. col.] An Ike rooter named Henry Clay.
1967		J. Breslin World of Jimmy Breslin (1968) 158: You can be a rooter. But you just a rooter in the stands and that’s all.
1991		in J. Breslin Damon Runyon (1992) 217: Horse Thief Burke performed another vital role, that of official rooter.