noun
stone-throwing fight; stone battle
Historical term for a mock battle or brawl using stones, often associated with children's play or local conflicts in pre-modern Japan.
昔の子供たちは石合戦をして遊んだという。
They say that children in the old days played at stone-throwing fights.
Compound of 石 (ishi, 'stone') and 合戦 (kassen, 'battle'), with rendaku voicing of かっせん to がっせん.