The term originates from the Jamaican/Reggae culture where a gunshot was used to signal/call a rewind of a big tune. Now, yelling BOH! or BWOH! is a common vocalization used to affirm the selections/sound of the Selectah/DJ.
Drum & Bass MC shouts at a party, “If you want the rewind let me hear you say BOH!” Crowd responds in unison, “BOH!”
Trainspotter standing all night with his arms folded right in front of the DJ booth finally hears an exclusive dubplate he recognizes, yells “BOH SELECTAH!” – and promptly raises his lightah!
taken from: www.urbandictionary.com