童心未泯

tóngxīnwèimǐn
(idiom) still a child at heart; to retain a playful spirit

CEDICT

one's childish disposition remains even though he/she is grownup

Wiktionary

年歲已大，卻仍保有兒童一般天真、純潔的心性。

重編國語辭典

年歲已大，卻仍保有兒童一般天真、純潔的心性

維基詞典

Entry Composition

1 童心 tóngxīn
  1. childish heart
  2. childish innocence
2 未 wèi
  1. not yet
  2. did not
  3. have not
  4. not
  5. 8th earthly branch: 1-3 p.m., 6th solar month (7th July-6th August), year of the Sheep
  6. ancient Chinese compass point: 210°
=3 泯 mǐn variant of 泯 mǐn
=3 泯 mǐn (bound form) to vanish; to die out; to obliterate