[yashap]

It’s a pretty broad term, but I’d define it as properties of a software system that make it hard to modify/maintain safely and easily. And it’s fixable, but takes an investment of time/effort/money to fix. The debt metaphor is that it can make sense to have a bit of this, but too much becomes crippling.

Often the most maintainable solution is simple and elegant, but it takes a lot of refactoring to implement, so a hacky, complex solution is implemented instead, because it’s faster/easier to implement. Such solutions tend to either contain bugs, or lead to bugs when built upon, and a lot of security vulnerabilities are basically bugs in hairy parts of systems that are hard to understand.