The lack of imagination in the definitions for this word is astonishing to me.
A fucktard is a metally and/or physically retarded person who is used by a non-retarded person for sexual gratification.
To call someone a fucktard is a vicious (and perhaps hilarious) insult because it is so spectacularly degrading. When you call someone a fucktard, you are not merely calling him or her a complete idiot, you are further implying that he or she is a kind of deranged whore who deserves the sexual degradation of being used as a sort of "human blow-up doll" (even as he or she might not even have the mental capacity to understand the sex acts that are performed on her or his possibly deformed body).
There is the additional repulsive dimension of possible sexual slavery on the part of the helpless or low-functioning retarded person.
People with Down's Syndrome might make especially good fucktards because they appear to be so affectionate, happy, and pliant.
Consider the following awful scenarios the next time you call someone a fucktard:
I'm so horny, but who has time to pick up women? I'm going to go down to the adult daycare and get me a fucktard.
I got a new fucktard that I keep locked up in my bedroom closet.
This new fucktard I found is blind in one eye, so I can pop out her glass eye and skull fuck her.