He has been eating abnormally little lately, maybe because of a stomach problem.

abnormally 是由形容词 normal（正常的）加上前缀 ab-（表示“偏离、反常”）和副词后缀 -ly 构成的副词。ab- 前缀表示“远离、不”，所以 abnormally 字面意思是“不正常地”，引申为“异常地”。