There was a reason for President Donald Trump’s particularly caustic response when a reporter asked him Wednesday about a new term coined about the president’s tariffs: TACO, or Trump Always Chickens Out.

He had not yet heard the term, according to a senior White House official who acknowledged to CNN that the president was caught off guard. Trump had said as much at the time, saying “I’ve never heard that” before calling it the “nastiest question.”

“He thought the reporter was calling him a chicken,” the official said, adding that Trump was “reasonably” frustrated with the phrase.

The acronym was coined in early May by a Financial Times columnist and is now used as shorthand by some on Wall Street to indicate that traders shouldn’t fret too much about Trump’s tariff threats, since he usually backs down.