They lawfully obtained ownership of this piece of land.

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