WASHINGTON — President Biden lashed out Thursday when asked why he’s called the “big guy” in an FBI informant file accusing him of accepting a $5 million bribe from a Ukrainian businessman while vice president.

“Why did the Ukraine FBI informant file refer to you as the ‘big guy,’ President Biden? Why is that term continuously applied?” The Post asked Biden at an event focused on reducing corporate junk fees.

“Why do you ask such a dumb question?” the 80-year-old president snapped.

An informant told the FBI in June 2020 that an executive at Ukrainian gas company Burisma confided that he bribed Joe and Hunter Biden and called the elder Biden the “big guy,” according to congressional sources who read the file.

Republicans say the nickname’s use is significant because it was not publicly known at the time. Nearly four months later, The Post reported in October 2020 that an email from first son Hunter Biden’s abandoned laptop said the “big guy” was due 10% in a business deal with CEFC China Energy.