An onomatopoeic adverb describing something fitting tightly or snugly with no gaps, or a small, crisp sound; used for clothing fitting closely or things closing tightly.
箱のふたがぴちっと閉まった。
The box lid closed snugly.
そのズボンは彼のウエストにぴちっと合っている。
Those pants fit his waist snugly.
雨戸をぴちっと締めて、風を防いだ。
I shut the storm shutters tightly to keep out the wind.