He can make you want to knock him down, if he feels like it, by simply saying "good morning". He possibly said simply "good morning" to Lord Culter. The difference was that, being his brother, Culter hit him.
In this excerpt from The Ringed Castle by Dorothy Dunnett, the character has a commanding presence that can provoke strong reactions from others just by a simple greeting like "good morning." His attitude and demeanor seem to influence how people respond to him, often leading to confrontations or conflicts.
Specifically, the quote hints at a tense relationship with his brother, Lord Culter. When the protagonist possibly greets him with the same casual "good morning," Culter's response is physical: he hits him. This shows a mixture of underlying hostility, familial tension, or personal animosity that manifests in aggressive behavior.
