gang of rowdies; band of ruffians
A somewhat literary or old-fashioned set phrase referring to a group of lawless, violent men. Often used in historical or dramatic contexts.
The town was ravaged by a gang of rowdies.
Composed of 無頼 (lawless, ruffian) + の (possessive particle) + 徒 (group, party). The exact historical derivation is uncertain, but the phrase has been used in literary Japanese to describe bands of outlaws.