Green’s Dictionary of Slang

(orig. US) a shout of encouragement, enthusiasm, e.g. directed at a musician.

1972		D. Claerbaut Black Jargon in White America 65: get it on v. an imperative expression meaning to start; commence; begin the activity: Get it on, man.
1977		N.C. Heard When Shadows Fall 159: [Big Red] grabbed Babs by the hair, lifted her face waist high and then punched the exposed angle of the jaw with a solid uppercut. [...] ‘Gititon, Big Red!’ someone else shouted.
1978		R. Caron Go-Boy! 190: Get up Buck and get it on!
2000		C. Cook Robbers (2001) 249: Someone near the back shouted, Get it on, Rufus!