plonked down (?)

шлёпнуться глаг.
общ. plop; flop; plonk; plunk; splash; tumble; plop down (Tony19); plump; fall with a wallop; thud; whop; flop down; drop (he dropped into the chair Рина Грант); collapse (to completely relax the muscles of your body because you are very tired, upset, etc. She came home from work and collapsed on the sofa. george serebryakov); flounce; squab
брит. come a cropper (or "purler" Liv Bliss)
диал., разг. wallop
идиом. come a purler (головой вперёд, во весь рост • "Only one thing, indeed, could have pleased me more – if I had been informed that Sir Watkyn Bassett had trodden on the soap and come a purler in the bathtub." (P.G. Wodehouse) ART Vancouver)
Макаров. go plash; go down with a wallop; plonk down; plump down; splash into
Макаров., разг. plunk down
разг. hit the ground with a thud (4uzhoj); pancake (Andrey Truhachev); take a spill (VLZ_58)
Gruzovik, разг. hit against (pf of шлёпаться); knock against (pf of шлёпаться); fall with a plop (pf of шлёпаться)

Интересно.

hit the ground with a thud удариться с глухим звуком о землю . Не подходит.

plop; flop; plonk
плюх плюх плюх.
Видимо, выбирать нужно из этих вариантов.

Поэтому я и предположила plonked down

She plopped down like that was her own couch. (?)
"Plop down on the couch" means to sit or lie down heavily and without much care, often indicating a relaxed or tired state. It suggests a somewhat casual and maybe even a slightly clumsy way of settling onto the couch.

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Валерий и Татьяна, спасибо за интерес к моему вопросу.

Uly, thank you so much!

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