Sunken eyes are eyes that appear sunken back into the eye sockets, they look a bit deflated. Normally chameleon eyes are very rounded and appear to be under pressure.
Sunken eyes are caused by dehydration. The chameleon does not have enough water in its body. As eyes are made mostly of water, this can be clearly seen in the eyes. It is a severe sign of dehydration and should be taken very seriously.
When you notice a chameleon as sunken eyes, it should be taken care of right away. It should be given water, either by misting or by forcing it to drink with a pipette. It is crucial that the chameleon gets some water in its system as soon as possible. A vet specialized in reptiles could also give the chameleon an IV with isotonic solution to get its body hydrated quickly.
The next step is to prevent this from happening again. Something is not working with the way you are supplying water to your chameleon. Apparently it does not drink from the water source you provide. Make sure you add another water source. You should also keep the environment much more humid by increasing the air humidity. If you stumble upon a chameleon with sunken eyes that is owned by someone else you should inform them that this condition is serious and that the chameleon needs water.