class Giraffe extends Neck {...} or something like that

The one with the 5m long nerve? Because it needs to loop around an artery near the heart, as that was the shortest way back when that nerve first developed. And now the source and destination are still close, but the heart moved. But no one has gotten around to make that legacy code more efficient.

One of my favorites as well.

It's mostly a software development term. Bugs are the most obvious kind of tech debt. They have to be fixed or a product will slowly become unusable over time, so when you release something with bugs you're incurring "debt" they must be "paid" later by fixing them. A lot of tech debt also involves corner cutting and bad design decisions that are hard to explain briefly.

Implementing a software feature takes a certain amount of time, and time translates pretty directly to money. Sometimes, you may need to meet a deadline or run out of budget, so you end up implementing the feature at a lower quality or without completing the usual checks or constructing it in such a way that nothing else can be built on top.

This allows you to meet the time/money constraints, but it will come back to haunt you, either making the implementation of future features more costly, or requiring the mess you left behind to be cleaned up, before trying to build on top.
As such, it feels a lot like you're taking up 'technical debt', which you'll have to pay for later.