Conor McGregor and Floyd Mayweather finally met in the boxing ring Friday ... but instead of throwing punches, the 2 fighters traded ferocious insults.

SSE Arena in London was packed for the 4th and final stop on the super fight world tour ... and Conor fans ruthlessly booed Mayweather as he tried to get his verbal jabs out.

Conor rubbed Floyd's "peanut head" and made fun of his "brittle hands" ... while Floyd dropped a homophobic slur and referred to Dana White as Conor's pimp.

It's the last time Floyd and Conor will meet for a while before the super fight in August. Which is probably a good thing, 'cause these guys gotta give their voices a break.