you (polite, to an older male); elder brother (polite)
Polite second-person pronoun used to address or refer to an older male, often in letters or formal speech. Can also mean 'elder brother' in a respectful way. Not used in casual conversation.
尊兄にはいつもお世話になっております。
I am always indebted to you, my elder brother (or respected older male).
Thank you for the wonderful gift you sent the other day.
Compound of 尊 (respect) and 兄 (elder brother). The exact historical derivation is uncertain, but the word has been used as a respectful term for an older male since classical times.