One who is both insane and stupid simultaneously. Such people do not have a brain as you or I know it. Instead, they only possess what it known to be a behavior pump: an organ which responds to any input with only simple, raw emotions. Indeed, we all possess such an organ but is kept in check by the higher functioning of the neocortex. MRI images of insanitards' heads usually show what looks like a full human brain, but upon further analysis the apparent gray matter serves as nothing more than a padded shielding to protect the behavior pump's sensitive feelings. Like zombies, A clean shot to the head is the only known way to permanently kill an insanitard.
Once a month, my girlfriend transforms into an insanitard.