Bhos

I remember there was a thread on Bo!

I suggest that Bo! is a derivative from bhos (Skt.)

bhos
interjection or voc. particle commonly used in addressing another
person or several persons. [from: bhavas; also cited in Pa_n.ini 8-
3,17]

While uttering abhiva_daye, after reciting the s'akha and gotra and
name, the saluting person says: asmi bhoh.