noun
A person who acts lawlessly or immorally; a somewhat literary or emphatic term for a wrongdoer.
彼は町で有名な不埒者だ。
He is a notorious scoundrel in town.
あの不埒者を捕まえろ！
Catch that villain!
ならず者 means 'rogue' or 'ruffian' and is more colloquial; 不埒者 is more formal and emphasizes moral lawlessness.
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'.