でんぐり返す

Compound of でんぐり (denguri) +‎ 返す (kaesu, “to turn something over”).^[1][2] The kaesu changes to gaesu as an instance of rendaku (連濁). Sometimes also pronounced without rendaku and with a hard k sound instead, as dengurikaesu.^[1]

The denguri part is of unclear origin. It only appears in this compound and derived forms.

• The den portion is most likely the same den found in terms such as でんでん (denden, “one after the next, rolling, continuous”, adverb), itself also found in the compound でんでん太鼓 (denden-daiko, “denden (rolling? continuous?) drum”, a small taiko drum with a handle extending from the bottom and two small balls attached by short strings to each side as strikers, played by holding in one hand and rapidly twisting the handle back and forth).
• The guri portion is almost certainly 繰り (kuri, “winding, reeling, spinning”), the 連用形 (ren'yōkei, “continuative or stem form”) of verb 繰る (kuru, “to wind, to spin, to reel”).

• (Tokyo) でんぐりがえす (Nakadaka – )^[2]
• IPA^(key):

でんぐり返(がえ)す • (dengurigaesu) transitive godan (stem でんぐり返(がえ)し (dengurigaeshi), past でんぐり返(がえ)した (dengurigaeshita))

1. to turn something over, to turn something upside-down
2. to make someone or something do a somersault (either forwards or backwards)

• (turn over): ひっくり返(かえ)す (hikkurikaesu)