A sinner is a person who has broken God’s rules. In church language, a sin is a wrong God does not want, and a sinner is the one who did it. The word is old and serious: the Bible, hymns, and priests use it a lot. Some teachings even say every person is a sinner, because nobody keeps every rule. Though actually, people also say it with a small smile — “I’m a sinner” can mean only “I did something a bit naughty,” like eating the last biscuit. That jokey use is common at the table; in a church the same word still lands as a heavy judgement. It is not a police word or a school word. You do not call a thief a sinner unless you are speaking in a religious way.