Their collaboration proved to be fruitful, resulting in a successful project.

fruitful 是由名词 fruit（果实）加上形容词后缀 -ful 构成的形容词。fruit 本义是"果实"，引申为"成果、收获"；后缀 -ful 表示"充满...的"。fruitful 字面意思是"充满果实的"，引申为"富有成果的、多产的"。