Otherwise spelled bumshaft, it is the anus. Affectionately known as the marmite highway, etc.
Hails from shaft, as in mineshaft (a deep hole in the ground), boom hails from bum, affectionately known as the arse, or anus.
I know your type! You like taking things up the boomshaft!
Olga is a real dirty bitch! She loves shoving things up her boomshaft!