expression, godan verb (-mu)
be knocked out; be overwhelmed; be flabbergasted
Idiomatic expression literally meaning 'to sink into the mat', used figuratively for being completely defeated, shocked, or at a loss for words. Often used in sports or competitive contexts, but can apply to any overwhelming situation.
彼のパンチで相手はマットに沈んだ。
His punch knocked his opponent out onto the mat.
その
He was completely floored by that question.