A person who acts lawlessly or immorally; a somewhat literary or emphatic term for a wrongdoer.
He is a notorious scoundrel in town.
Catch that villain!
不埒 (lawlessness, impropriety) + 者 (person). 不埒 is a Sino-Japanese word meaning 'unreasonable' or 'outrageous', and 者 is a common suffix for 'person'.