The child has a freakishly good memory for his age.

freakishly 是由名词 freak（怪人、怪事）加上后缀 -ish（具有...特性的）变成形容词 freakish（怪异的），再加上副词后缀 -ly 构成的副词。freakishly 表示"反常地、惊人地"，强调超出正常范围的程度。