n. a mature person who acts immature. The person wears an immaturity shell to hide their true personality, either out of insecurity or detest of other mature persons. This immature shell protects the secretly mature person from responsibility and from the company of dull people.
Emily was shocked to discover that her boyfriend was intelligent and responsible, even though he acted foolish the majority of the time. It became apparent that under his jackass shell, he was a maturtle.