Webster’s Revised Unabridged Dictionary · 1913
Root
Root, v. i. [Cf. Rout to roar.] To shout for, or otherwise noisly applaud or encourage, a contestant, as in sports; hence, to wish earnestly for the success of some one or the happening of some event, with the superstitious notion that this action may have efficacy; — usually with for; as, the crowd rooted for the home team. [Slang or Cant, U. S.]