1Inexcusable; unjustifiable — a somewhat formal way of describing an act or treatment as morally unacceptable.
彼の裏切りはすまない行為だと皆が言った。
Everyone said his betrayal was an inexcusable act.
弱い者いじめをするなんて、すまない話だ。
Bullying someone weaker than you is simply unjustifiable.
2Sorry; remorseful — describes the feeling of regret toward someone, commonly used in casual spoken Japanese.
遅れてしまって本当にすまない。電車が遅延していたんだ。
I'm really sorry I'm late — the train was delayed.
君に迷惑をかけてすまないと思っている。
I feel bad for causing you trouble.
大事な会議を忘れてしまい、上司にすまない気持ちでいっぱいだった。
I forgot the important meeting and felt deeply sorry toward my boss.
3Used as an interjection to briefly express an apology, or sometimes gratitude. A casual, somewhat masculine-sounding alternative to すみません.
すまない、手伝ってくれて本当に助かったよ。
Thanks — you really helped me out. (literally: Sorry — that helped a lot.)
道をふさいでしまってすまない。すぐどきます。
Sorry for blocking the way. I'll move right away.
すまないが、少し静かにしてくれないか。
Sorry, but could you keep it down a little?