A milkman is, in Britain especially, the person whose job was to bring bottles of milk to people’s houses early each morning. For a long time this was ordinary British life: a quiet little van called a milk float, the clink of bottles on the step, a note left out if you wanted extra. That round has mostly gone; shops took it. American English knows the idea, but it was never as central there. The word still sounds British and a bit nostalgic — something grandparents mention. He does not milk the cow. He delivers what is already in the bottle.