As a joke a couple of times Dearka calls Yzak “Lord Commander” to get a rise out of him. Specifically, “taichou-dono.”

"-dono roughly means “lord” or “master”, and lies below sama in its respect level. It’s a title that has almost gone out of use in daily conversation. When it is used, it is usually as a joke expressing an exaggeration of age."

So essentially Dearka is calling Yzak a cranky old man.