puer aeternus
/ˌp(j)uːər ɪˈtəːnʊs/ /ˌp(j)uːər ʌɪˈtəːnʊs/ /ˌp(j)uːər iːˈtəːnʊs/(also puer aeternis, puer eternis, puer eternus)
noun
In Jungian psychology: an archetype of eternal youth. Also (more loosely): an emotionally or psychologically childlike man.
Origin
Early 20th century. From classical Latin puer aeternus, after use in a German context by C. G. Jung.