Donald J. Trump @realDonaldTrump · 14h

Very sad to see States allowing roving gangs of wise guys, anarchists & looters, many of them having no idea what they are doing, indiscriminately ripping down our statues and monuments to the past. Some are great works of art, but all represent our History & Heritage, both….

Wise guys is Mafia slang for a "made man", someone who has taken the Mafia oath and confirmed their loyalty by committing murder.

So… POTUS is warning us that organized crime operatives are in these "protests" and folks should be careful. He may be using "wise guy" to refer to organized crime groups in general rather than the Mafia specifically. All the groups require some crime to be committed as part of their initiation, usually murder.