どっかと

adverb

with a thump; floppingly

Mimetic adverb describing a heavy, dull impact or the sound and motion of something flopping down heavily.

• 疲つかれてソファにどっかと座すわり込こんだ。

  Exhausted, I flopped down onto the sofa with a thump.

• 大おおきな荷に物もつを床ゆかにどっかと置おいた。

  He put the heavy luggage on the floor with a thud.