in cheers, 1870, a shortening of hurrah. Adjective rah-rah is attested from 1907, originally indicating college life generally, later enthusiastic cheerleading.
Entries linking to rah
hurrah interj.
1680s, apparently an alteration of huzza; it is similar to shouts recorded in German, Danish, and Swedish; perhaps it was picked up by the English soldiery during the Thirty Years' War. Hurra was said to be the battle-cry of Prussian soldiers during the War of Liberation (1812-13), "and has since been a favourite cry of soldiers and sailors, and of exultation" [OED]. Hooray is its popular form and is almost as old. Also hurray (1780); hurroo (1824); hoorah (1798). As a verb from 1798. American English hurra's nest "state of confusion" is from 1829.