Green’s Dictionary of Slang

(US campus) to have a good time; to act utterly madly, drunkenly, obstreperously.

1969		Current Sl. III:4 9: Rally, v. To attend a party; drink; to have a ‘wild time.’.
1975		G. Underwood ‘Razorback Sl.’ in AS L:1/2 64: Let’s go to Maxine’s and rally after the game.
1982		W. Safire What’s The Good Word? 304: I might rally later when my roomies flush me for a ‘za ra.’.
2004		Eble Campus Sl. Apr. 5: rally – get energy, join in activity with others.
2016		C. Eble (ed.) UNC-CH Campus Sl. Spring 2016 8: RALLY — gain or regain strength to party: ‘Time to rally! We’re going out!’.