It’s from Working Effectively with Legacy Code. I don’t recall the exact definition but it’s something to that effect. Legacy = lack of automated tests.

The biggest barrier to changing code is usually insufficient automated testing. People are terrified of changing code when they can’t verify the results before breaking production.

More glibly legacy code is “any code I don’t want to deal with”. I’ve seen code written 1 year prior officially declared “legacy” because new coding standards were being put in place and no one wanted to update the old code to match.