old boy [countable]
1 British EnglishSES
a man who used to be a student at a school:
an old boys' reunion
the system by which men who went to the same school, belong to the same club etc use their influence to help each other
3 British English spoken
an old man:
He's a nice old boy.
4 British English old-fashioned
used when speaking to a male friend:
How are you, old boy?