Green’s Dictionary of Slang

1. anything or anyone extreme, violent, highly aggressive.

1850		Man of Pleasure’s Illus. Pocket-book n.p.: This harpy is said to have been a rooter to the poor shakes, who beat stones for their doss, bub, and grub.
1920		Bulletin (Sydney) 24 June 20/2: The straight-ahead prad, no matter with how great a jar he hits the ground, is a mere ‘rooter’ or ‘pig-jumper’.

2. anything considered excellent, first-class.

1860		Hotten Dict. Modern Sl., Cant etc. (2nd edn) 201: ROOTER, anything good, or of a prime quality; ‘that is a rooter’, i.e., a first-rate one of the sort.
1873		Sl. Dict.