This is the 4744th most frequent Japanese word.
松夫
"松夫" (Matsuo) is typically used as a Japanese male given name, meaning "pine man" or "pine husband."
Here, '松夫' is used as a proper noun, referring to a person's name, specifically Matsuo.
松夫さんは親切な人です。
Matsuo-san is a kind person.
In this sentence, '松夫' again serves as a proper noun, designating ownership as Matsuo's garden.
松夫の庭には美しい松の木があります。
Matsuo's garden has beautiful pine trees.
Here, '松夫' is used as the name of the person to whom the letter was written, following Japanese grammar rules for addressing people.