noun
A forward or backward roll; often used in contexts like gymnastics, play, or falling.
子供がもんどりを打って遊んでいる。
The children are playing by doing somersaults.
彼は階段からもんどりを打って落ちた。
He tumbled head over heels down the stairs.
でんぐり返し is a more common word for somersault, especially in casual speech. もんどり can sound slightly more dramatic or literary.
The origin is uncertain. The kanji 翻筋斗 is a jukujikun reading, where the characters mean 'turn', 'muscle/tendon', and 'dipper/ ladle', but the reading もんどり is not derived from the individual kanji sounds.