ramrod; cleaning rod
A rod used for cleaning the barrel of a firearm or cannon. A specialized historical/military term.
彼は洗い矢で銃身を掃除した。
He cleaned the gun barrel with a ramrod.
Compound of 洗い (arai, 'washing') + 矢 (ya, 'arrow'), from the rod's shape and cleaning function. The exact historical derivation is uncertain.