I first heard this term from my mother, who over the years has immersed herself in meditation, yoga, and Buddhist philosophy. Rather than referring to a Roshi, Zen Master can be used in a more humorous manner by referencing that difficult person in your life.
Your everyday Zen Master tests your patience and may not be aware of doing so. Unless you want an endless battle with this person, you are left with no choice but to figure out the best way to deal with them. Once you're reached a certain level of calmness and understanding, you might try and slip the term into conversation, just for fun.
Co-worker 1: I know that I'm going for lunch so I won't be here to hear the radio, but could you turn down the radio? Because I don't like it playing that loud even though I won't be here to hear it.
Co-worker 2: But you're going on your lunch break . . .
Co-worker 1: I know but could you turn it down?
Co-worker 2: Oh Sharon, you are my Zen Master.
Co-worker 1: Thank you Kaitlyn. I know I am!!
Since becoming familiar with the term, I try and remember that everyone has something to teach us, especially those who challenge us. Perhaps that obnoxious person is doing something we ourselves have done in the past and, rather than feeling angry, we should try and empathize. When we show compassion to others, we are being compassionate to ourselves as well.
Of course, it can also feel pretty good to just let it rip! After all, we can't expect people to change if we don't give them something to think about.