一干二净

yī gān èr jìng

completely; clean as a whistle

Character by character

一yī
one

干gān
dry

二èr
two

净jìng
clean

他把学过的东西忘得一干二净。

He's completely forgotten everything he learned.

Tone: neutral.

Usually a complement: 忘得, 吃得 or 花得一干二净.

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