Green’s Dictionary of Slang

rally v.

[? SAmE pep rally; but note 18C SE rally, to banter with, to tease with pleasantry]

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

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