不知天高地厚

bùzhītiāngāodìhòu ㄅㄨˋ ㄓ ㄊㄧㄢ ㄍㄠ ㄉㄧˋ ㄏㄡˋ

    • not to know the immensity of heaven and earth
    • an exaggerated opinion of one's own abilities

Character by character

Saying俗语俗語

Idiom惯用语 · In everyday use

Literally
not knowing how high heaven is or how thick earth is
Meaning
Ignorant and arrogant; not realising one's limits or the seriousness of things. Critical.

Sources