(someone else's) elder brother; older brother
Honorific term used to refer to another person's elder brother, showing respect. Not used for one's own brother.
I hear that Mr. Tanaka's elder brother is a doctor.
Where does your elder brother live?
Compound of 兄 (あに, 'elder brother') and 御 (ご, honorific prefix/suffix). The exact historical derivation is uncertain, but it follows a pattern of adding 御 to kinship terms for respect.