ボコッ

ボコッ, ぼこっboko

adverb

  1. with a thwack
  2. wack
  3. thump
  4. conspicuously (of a hole, bulge, dent, etc.)
  5. gapingly
  6. yawningly
  7. (protruding) noticeably

Study it in Yanamino

Data from JMdict, KANJIDIC2 (EDRDG), KanjiVG, CC-CEDICT and Make Me a Hanzi. Sources and licences