As the only legitimate heir, he lawfully inherited the property.

lawfully 是形容词 lawful（合法的）加上后缀 -ly 构成的副词形式，表示"合法地、依法地"。lawful 由名词 law（法律）加上形容词后缀 -ful（充满...的）构成，字面意思是"符合法律的"。