Primarily a saying derived from contact sports terminology. Refers to one player blowing the living shit out of another with a solid check, block or tackle. The recepient of said play is usually badly shaken up, from the sound of contact just as much as the actual contact itself. Conventionally, the player who delivers the blow is usually a bit bigger than the recepient. Hence the term "lowering the boom". Some people who have been known to lower the boom are Brian Urlacher, Dwight Freeney, Chris Pronger, Scott Stevens, and even Ben Wallace (one crazy ass motherfucker).
Brian Urlacher: Next time Marvin Harrison cuts into the centre like that on a square in route, I'm lowering the boom on that bugger
Ben Wallace: Foo! You think you gonna drop that fade-away on me, mofucka? Ima lower the boom on you evry time, punk ass foo!
It's a very phrase to describe a child raper. Someone who likes to rape children and goes to many locations
Beware of him and if you see him, DO NOT GO NEAR HIM