I would say that a thrashed car is one that shows multiple signs of excessive wear.
At some point in a cars life they start smoking, bad habit, obviously cars can't read the warnings. But the smoking may not be thrashed, just used. But when you couple that with a gearbox that doesn't want to engage a gear properly, you tend to start looking at the character behind the wheel.
So in my opinion its an accumulation of things and its a subjective term at the very best.
Regards,
Justin