adverb, adverb taking the 'to' particle
Mimetic word describing a heavy object falling or dropping with a dull, solid sound. Often used with と.
大きな石がボトリと落ちた。
A large stone fell with a thud.
彼は疲れて、ベッドにボトリと倒れ込んだ。
Exhausted, he collapsed onto the bed with a thud.
Onomatopoeic formation; the exact origin is uncertain.