noun
battle royal; large-scale free-for-all
Refers to a chaotic, large-scale fight involving many participants, often with no clear sides or rules. Used in sports, games, or metaphorical contexts.
試合は最後に大乱戦になり、観客は大興奮だった。
The match turned into a huge free-for-all at the end, and the crowd was thrilled.
ゲーム内で大乱戦が発生し、誰が敵か味方かわからなくなった。
A battle royal broke out in the game, and it became impossible to tell friend from foe.
混戦 implies a confused, mixed-up fight where sides are unclear; 大乱戦 adds the nuance of grand scale and wildness.
Compound of 大 (dai, 'big') + 乱戦 (ransen, 'chaotic battle'). The exact historical derivation is uncertain, but the word follows a common pattern of intensifying a noun with 大.