A.. (ahem) mentally handi-capable (that being the 'politically correct' term,) individual, whom, even thouhg they may be unable to say the most basic of sentences, are geniuses at certain areas. And idiot savant can play the piano like a pr0, but may not be able to say his/her own name. Other such areas include painting, ect.
Peter Griffin is an idiot savant.
Though he is mentally handi-capable, he can play the piano beautifully (albeit only when he is drunk).
A person who (unlike a true savant in that s/he is not medically considered mentally disabled or autistic) is extraordinarily gifted in one or two specific areas, yet is still incapable of handling simple, easy tasks or common-sense situations. In other words, an idiot savant is an incredibly intelligent ditz.
Wow, that girl can't tell left from right, but she aced that super hard biology test! What an idiot savant!