Bhai sahib is a Punjabi and Sikh title of veneration given to a male; the word "Bhai" means "brother" and "sahib" means "Sir". It is an honorific stemming from historic times. The founder of Sikhism, Guru Nanak used the title "Bhai" for many figures including Bhai Mardana, Bhai Lalo, etc.
"Sahib" is a Punjabi term of respect, meaning Sir, master or lord, etc. It is from the Arabic language passed on to several languages including Hindi-Urdu (Hindustani), Bengali and Marathi, etc. It has also been translated as: grace or, as in the Sikh religion, "Guru's honor." It comes from the Arabic ṣĝḥib صاحب, originally "friend, companion" (from ṣaḥiba صحب "he accompanied"). Its grammatical feminine form is ṣĝḥibah صاحبة, but the sue may differ greatly.