To hit something so hard that it no longer exists in its original form. To break something to the point that it is almost unrecognizable.
Boom = the impact of the hit, Smush = the condition of what got hit!
"Man, I hit dude in the face so hard that his glasses just boom smushed on his face!"
"The other day I saw this car accidnet and the whole front of one car just got boom smushed!"
"Keep talkin shit and I will boom smush your a$$!"