3:48 PST -- A rep for Mc D's tells us they've determined the hack was from an outside source and not from within the corporation.

Donald Trump can safely order Big Macs again -- McDonalds' vicious Twitter attack against him was the work of hackers ... according to the burger giant.

Micky D's verified account tweeted Thursday morning, "You are actually a disgusting excuse of a President and we would love to have @BarackObama back, also you have tiny hands."

It was posted for at least 30 minutes before McDonald's deleted it and said Twitter confirmed the company's account was compromised. McDonald's says it's now investigating.

Bottom line: No McDonald's beef with Trump ... although he does enjoy their burgers.