People afflicted with this disease often cannot be troubled to open a dictionary or get around to actually completing the second grade.
See also: Paris Hilton Retarded
Principal: Ma'am, we prefer the term Dictionarally Challenged...and yes, he's a 'tard.
Job Applicant: Weight, your telling me your not hiring me because I live in that they're trailer?
Manager: No, I'm not hiring you because you're a dumb ass.