No? Neither did I.
I was fortunate to visit Porto on the day of the São João Festival (23rd/24th June) where what seems like the entire city comes down to the river to pay tribute to Saint John the Baptist. Part of these celebrations involve hitting each other with plastic hammers as you can see from a video I took below:
As you walk along, random people you don’t know hit you with a plastic hammer and it is perfectly acceptable for you to hit people too.
I was told by somebody that the original tradition was hitting people with onions in a bag and has somehow morphed into noisy plastic hammers, presumably as these do not hurt as much as an onion might. Rather than onions, I have also read elsewhere that it actually started with hitting each other with Leeks. Why? No one seems to know.