Sexual Nirvana: The point of extreme sexual satisfaction and/or gratification which leaves you unable to speak properly, think clearly, or function correctly in even the simplest of tasks. (Eg: Talking is impossible, as you are left staring blankly into nothingness.)
The State of mind that you are left-in after experiencing the MOST INTENSE ORGASMIC BLISS.
Someone who is 'Cum-Dumb' could very easily walk right off a cliff without even noticing because all they can think about is their intense orgasm, and nothing of potential dangers around them.
Someone who is 'Cum-Dumb' should NOT operate heavy machinery, or drive any sort of vehicle, etc.