An onomatopoeic adverb indicating a sudden jerk, drop, or sharp change (physically or figuratively).
発表の直後、株価がガクンと下がった。
Right after the announcement, the stock price plunged suddenly.
階段で足を踏み外し、膝がガクンと落ちた。
He slipped on the stairs and his knees buckled with a jerk.
疲れて集中力がガクンと落ちてしまった。
I got tired and my concentration dropped sharply.