一窍不通

一竅不通
yīqiàobùtōng
  1. lit. doesn't (even) enter a single aperture (of one's head)
  2. I don't understand a word (idiom)
  3. it's all Greek to me

CEDICT

to not understand a thing about the topic; it's all Greek to me

Wiktionary

一个心窍不通。语本《吕氏春秋．贵直论．过理》：「其窍通，则比干不死矣」句下汉．高诱．注：「故孔子言其一窍通，则比干不见杀也。」后用以比喻人昏昧不明事理，或对某事完全不懂。《醒世恆言．卷三五．徐老僕义愤成家》：「这萧颖士又非黑漆皮灯，泥塞竹管，是那一窍不通的蠢物。」《文明小史．第四二回》：「但是人数多了，自难免鱼龙溷杂，贤愚不分，儘有中文一窍不通，借着游学到海外玩耍的。」

重編國語辭典

比喻人昏昧不明事理，或对某事完全不懂

維基詞典

Entry Composition

1 一 yī
  1. one
  2. single
  3. a (article)
  4. as soon as
  5. entire; whole; all; throughout
  6. "one" radical in Chinese characters (Kangxi radical 1)
  7. also pr. yāo for greater clarity when spelling out numbers digit by digit
2 窍 qiào
  1. hole
  2. opening
  3. orifice (of the human body)
  4. (fig.) key (to the solution of a problem)
3 不通 bùtōng
  1. to be obstructed
  2. to be blocked up
  3. to be impassable
  4. to make no sense
  5. to be illogical