A substitute word or phrase that, while constructed to be socially more acceptable, drastically subtracts from the legitimate intent and purpose of the original.
So, how do we go from 'shell-shock' to 'battle-fatigue', from 'old dude' to 'senior citizen', from 'rape victim' to 'unconsenting intimacy participant'? Because our culture thrives on caustic euphemism: The dumbed-down, homogenized, neutralized language of the mindless majority who want it that way...
A vagina that is perceived to be dirty, diseased, or in some manner toxic. Also sometimes referred to as the Caustic Cleft.
When a guy hooks up with the last remaining (and often disgusting) woman in the bar he will often tell his friends, "I am going deep into the Caustic Valley tonight boys."