Smash Gash is a catch combination of Smash "The action/process of fucking someone good"
and Gash "A term for the female genitalia, implying that it looks like a hatchet wound"
Smash Gash is just a catchy modern way of saying "Pound Vag"
but has a certain catchy ring to it that sounds a million times better,
and is one of the many terms invented by TheBennji