It is the feeling of pure retardation after an orgasm. You are unable to communicate or operate your body.

When one orgasms so hard that they make retarded faces and are unable to function properly afterwards

A cumtard is a shortening of the phrase cum drinking retard.

Used as a way of letting someone know that you consider them to be a retarded cum receptacle.

One who is dumbtardedly infatuated with mansyrup / mammalsauce.

A very retarded person who is also very horny