Its meaning outside of taxes is something which has dropped to zero value. So if you damage your car beyond repair, for example, that would be a "write off". It means it's not worth the repair costs because it's cheaper to buy a new one.
Right. What it doesn't mean is that the thing being written off is valueless, though. I've seen several perfectly safe and drivable cars written off because of cosmetic damage that would have cost more to fix than the car was worth. But the cars were otherwise fine.
Except this part:
> because it's cheaper to buy a new one.
is not true. A write-off is because the repairs exceed the fair market value of the thing being written off. But the thing is used, not new. The fair market value is likely to be well below the cost of replacing it with something new.
> It means it's not worth the repair costs because it's cheaper to buy a new one.
No, it just means insurers are assholes rigging the game.
A simple example: my personal MacBook broke. MacBooks are written off in 5 years. My insurer only wants to pay the surplus value (€200).
I tell them okay, instead of the €200 find me a replacement MacBook of the same model and year with approximately the same config. “Sorry sir we don’t do that.”
Okay, do they think I can find the same MacBook for €200? “Probably not sir..”