The Brief: Go off is a phrase used to egg someone on.

DEEP DIVE
Telling someone to “go off” is usually a way to praise them and to encourage them to keep doing what they’re doing. “Going off” can refer to talking about something or doing something. The phrase can also be used more vaguely, usually as positive reinforcement, but sometimes sarcastically or negatively. The term is attributable to African American Vernacular English (ebonics). Currently, it’s popularly used among Kpop stans.
i love hearing my man gossip/vent. like go off babe lmk what’s bothering you. 😌
— kaymiah 👑 (@iTweetDope_ish) January 13, 2019
YES GO OFF PARK WOOJIN SHOW EM WHO’S KINGGGG pic.twitter.com/9dcEjxG0Gu
— 3PARK💘 (@o_h_h9599) January 15, 2019
Watching and cheering on Melissa as she shuts down the trash men in her mentions…GO OFF SIS pic.twitter.com/sL4BaasTOw
— bailey (@emilia_cIarkes) January 15, 2019
But Go Off I Guess
The phrase “but go off I guess” comes from Tumblr. It’s usually used after contradicting someone or calling them out. It can be meant seriously or sarcastically.
no i love /you/ more but go off i guess
— blake belladonna (@blaketxt) January 15, 2019
You probably shouldn’t take a class that’s part of the gender studies minor if you’re sexist and transphobic, but go off I guess
— rachael🐝 (@rachaelcwilson) January 15, 2019