妄自尊大

wàngzìzūndà
ridiculous self-importance (idiom); arrogance

CEDICT

to be arrogant; to be self-important; to suffer from megalomania

Wiktionary

骄矜自大，自命不凡。

重編國語辭典

傲慢、自视甚高

維基詞典

Entry Composition

1 妄 wàng
  1. absurd
  2. fantastic
  3. presumptuous
  4. rash
2 自尊 zìzūn
  1. self-respect
  2. self-esteem
  3. ego
  4. pride
3 大 dà
  1. big; large; great
  2. older (than another person)
  3. eldest (as in 大姐 dàjiě)
  4. greatly; freely; fully
  5. (dialect) father
  6. (dialect) uncle (father's brother)