不知天高地厚

bùzhītiāngāodìhòu
  1. not to know the immensity of heaven and earth
  2. an exaggerated opinion of one's own abilities

CEDICT

to have an exaggerated opinion of one's abilities; to think one knows everything; to be ignorant

Wiktionary

懵然无知而狂妄自大。《红楼梦．第一九回》：「再不说了。那原是我小时不知天高地厚，信口胡说，如今再不敢说了。」《儿女英雄传．第三四回》：「如今年过知非，想起幼年这些不知天高地厚的话，真觉愧悔。」也作「不知天高地下」。

重編國語辭典

懵然无知而狂妄自大

維基詞典

Entry Composition

1 不知 bùzhī
  1. not to know
  2. unaware
  3. unknowingly
  4. fig. not to admit (defeat, hardships, tiredness etc)
2 天高地厚 tiāngāodìhòu
  1. (figurative) profound; deep (of kindness)
  2. complexity of things
  3. profound; deep (of kindness)
  4. the vastness of the world