反义词前缀是im+mature，中文意思是adj. 发育未全的（人）幼稚的，不成熟的粗糙的

His speech is immature， his vocabulary limited.

他讲话不成熟，词汇量也有限。

He was a young fellow， very green， very immature.

他是个年轻的家伙，没什么经验，很不成熟。

He said that we were emotionally immature.