noun
irregularly placed stakes (defensive obstacle)
Historical military term for stakes driven into the ground or underwater at irregular angles and intervals to impede enemy movement. Typically encountered in descriptions of fortifications or castle defenses.
城の周囲には乱杭が仕掛けられていた。
Irregular stakes were set up around the castle.
水中に乱杭を打ち込んで敵船の侵入を防いだ。
They drove irregular stakes into the water to prevent enemy ships from entering.
Compound of 乱 (ran, 'disorderly, irregular') and 杭 (kui, 'stake, post'), with rendaku voicing of the second element. The term directly describes the irregular placement of the stakes.