What is hard licker?

It is an old expression that means Alcoholic Beverages. The Word liquor originally was used to mean any hot liquid like soup or broth. Since alcoholic beverages were "hot" going down, the name was applied because you could get drunk from them. There is also the expression "pot liquor", meaning a boiling liquid.

Beer wasn't considered "hard" because most people drank it. It was easy to brew, and didn't cost much.