n.person who is mentally retarded in general but who displays remarkable aptitude in some limited field (usually involving memory)
英汉双解大词典
idiot savant/ˈiːdjəʊ sæˈvɑ̃, ˈɪdɪət ˈsævənt/
1.
N（名词）
a person with learning difficulties who performs brilliantly at some specialized intellectual task, such as giving the day of the week for any calendar date past or present 白痴学者