Tekashi 6ix9ine might be on probation, but the dude isn't afraid to fire back if someone tests him out in public -- which is exactly what happened in Vegas this weekend.
The rapper showed up Saturday to the T-Mobile Arena for UFC 266 ... and as he was making his way in and past fans behind barricades -- he had a little face-off with someone who was NOT a fan.
Check out this video TMZ obtained of the altercation -- you see 69 ignoring the taunts from this dude, who's calling him a bitch and a ton of other insults.