unarmed; without a sword
Literally 'without a waist-sword'; used in historical or samurai contexts to mean unarmed or not carrying one's sword. Rare in modern Japanese.
侍が無腰で城下を歩くことは許されなかった。
Samurai were not allowed to walk around the castle town unarmed.
Compound of 無 (mu, 'without') and 腰 (koshi, 'waist'), referring to the absence of a sword worn at the waist. The reading むごし is a regular on-kun combination.