The phrase is used as an insult or an interjection expressing disgust or anger. Derived from the Jamaican word bomboclaat, bomboshart is a combination of bomboclaat and shart.
Joe: Hey, where have you been all day?
John: Just finished up in the washroom.
Joe: “BOMBOSHART”
Made-up semi-serious semi-joke curse word. Undoubtedly comes from Jamaican curse phrase bomboclaat. Used by Roots Manuva on his album Brand New Second Hand.