ガクッ

adverb, adverb taking the 'to' particle

with a collapse; with a slump

Mimetic adverb describing a sudden loss of strength, causing something to collapse, slump, or droop. Often used with と and verbs like なる or する.

• 疲つかれてガクッと膝ひざをついた。

  Exhausted, I slumped to my knees.

• 人にん形ぎょうの首くびがガクッと前まえに倒たおれた。

  The doll's head drooped forward with a slump.