Wersh means – very sour and at the same time dry. So drinking something which is wersh has the opposite effect from what you would want.

It draws your mouth in and generally makes you screw your face up; not nice. It certainly does nothing for a thirst.

Apparently, some people use the word wersh to mean something which is edible but has no taste. I’ve never heard it used in that way, but it might be a regional difference.