An onomatopoeic adverb meaning 'with a thud' or 'with a heavy thump,' describing the sound or manner when something heavy falls or someone collapses down.
箱をどさっと床に置いた。
He set the box down on the floor with a thud.
本が棚からどさっと落ちた。
Books fell off the shelf with a thud.
彼は疲れてソファにどさっと座った。
He flopped down on the sofa, exhausted.