Who is a Zen master?

Zen Master is the person who teaches Zen Buddhism to others.

However "teaching" sounds rather benign and images of sitting down and listening to a wise elder give instruction fill the mind. An example of Zen teaching by the owner of a teashop:

The Zen master Hakuin used to tell his students about an old woman who owned a tea shop in the village. She was skilled in the tea ceremony, Hakuin said, and her understanding of Zen was superb. Many students wondered about this and went to the village themselves to check her out. Whenever the old woman saw them coming, she could tell immediately whether they had come to experience the tea, or to probe her grasp of Zen. Those wanting tea she served graciously. For the others wanting to learn about her Zen knowledge, she hid until they approached her door and then attacked them with a fire poker. Only one out of ten managed to escape her beating.