Seahawks' Jaxon Smith-Njigba Curses On Referee's Hot Mic, 'That's Some Bulls***!'

Jaxon Smith-Njigba let everyone watching "Thursday Night Football" know he was pissed he was flagged during a critical 4th quarter drive, calling it straight up BS ... in a moment that was captured on an open microphone.

Whoops.

The expletive was heard loud and clear, on both the stream of the game, and in the stadium, as the Seahawks star and his squad played the Cardinals in Arizona in a week 4 matchup.

Up 11 in the 4th quarter, and in the red zone, Smith-Njigba was flagged for holding ... and he made his frustrations with the penalty very clear.

"That's some bulls***!" he yelled.

The official's reaction was priceless ... he gave JSN major side-eye as he realized the bad word would be heard by the millions.

Al Michaels and Kirk Herbstreit, the Prime broadcasters, weren't clutching their pearls after the four-letter was uttered ... they laughed, but also advised JSN to be careful, as he could've been penalized further.