On the record, drink somewhere else. Avoid mugs like that. Off the record loudly ask him what his problem is and tell him never to come near you again. Then he'll either over react and then you can act defensively or he will never speak to you again. Sneaky wise, without being technical just either grab his nuts hard and squeeze or bend his little fingers back hard. Or scrape your heel down his shins. no need to be technical. There are two points in the neck and one in the upper arm i could mention to pressure but they're not really covert. Used for moving on protestors in my job! Or you could discreetly knee hard into upper outer thigh high or low on inner thigh. Wouldn't get noticed in a crowd. Or way off the record catch up with him outside and get rid of the problem. But i wouldn't ever advise that for a million reasons. Problem is all this could cause more problems and a hurt pride can cause an idiot to return at the head of fifteen fellas and then you've gotta have a fight. Best avoided altogether mate.