What it means
It means doing more than you really had to, usually because you care about getting it right or helping someone out. You use it for the person who could've done the bare minimum and dipped, but stuck around, sorted the chaos, and put a bit of heart into it. It's not hardcore slang, more everyday English, especially at work.
Usage examples
"Dave clocked off, saw the printer having a meltdown, and stayed to sort it, then nipped Tesco for biscuits. Proper legend, always going the extra mile."
"The plumber went the extra mile, fixed the leak and then mopped the whole floor without me even asking."
"She didn't just send the files over, she cleaned up the whole deck and fixed the typos too. Proper went the extra mile there."
"Big up to the taxi driver, he saw Nan was struggling with the bags and carried them right to the door. Man went the extra mile."
"I only asked for the report, and she came back with the figures cleaned up, notes added, and a tidy summary on top. Proper went the extra mile."
Where it comes from
It goes back to Matthew 5:41 in the King James Bible, where Jesus says if someone compels you to go one mile, go with them two. The old image was about doing more than you're forced to do. Later it settled into everyday English for putting in extra effort out of care or decency.
