i-adjective
dissolute; loose conduct
Rare, literary expression describing someone whose behavior is unrestrained or morally loose. The phrase is a set expression and not freely modifiable.
彼は若い頃、素行が修まらなかったと聞いている。
I hear that in his youth, his conduct was loose.
A more common and modern way to say someone's behavior is bad or delinquent, without the literary or archaic feel of 素行が修まらない.
The expression combines 素行 (conduct) with the negative form of 修まる (to improve, to be corrected). The exact origin is uncertain, but it is a rare, fixed phrase.