imprisonment (for 1-3 years; third most severe of the five ritsuryō punishments)
「徒」 has other readings with different meanings.
ordinary; common; usual
vain; futile; transient; frivolous
foot soldier (Edo period); samurai on foot
useless; vain; aimless; idle
five punishments (of ancient China: tattooing, cutting off the nose, cutting of…