An onomatopoeia representing a heavy thud or impact (the sound of something heavy dropping or hitting); can also express a feeling of collapse. Often used as 'dosun to' adverbially.
箱が床にドスンと落ちて、中の荷物が散らばった。
The box thudded onto the floor and the contents scattered.
彼は大きな荷物を置いて、ドスンと椅子に座った。
He set down the heavy luggage and plopped down into the chair.
試験に落ちて、気持ちがドスンと沈んだ。
Failing the exam made me feel suddenly downhearted.