What does the idiom highway robbery mean?

The idiom "highway robbery" means charging an exorbitant or unfair price for something. It implies that the price is so high it's like being robbed on the side of the road.

Here are some examples of how it's used:

* "That restaurant charged $30 for a burger! That's highway robbery!"

* "They're trying to sell this used car for $10,000? That's highway robbery."

The idiom originated from the practice of bandits and thieves robbing travelers on highways in the past. The idea is that the price being charged is so unreasonable that it feels like being robbed.