Chinese idiom · 成语
乘虚而入
Meaning
To enter through a gap; to exploit weakness.
Character by character
Origin and story
Literary usageThe phrase uses the military-looking image of entering where defenses are absent. An exact attestation occurs in Zhang Junfang's Song anthology Yunji Qiqian (雲笈七籤), volume 120, in an account concerning Lu Wei: an entry is made through the rear wall when the way is unguarded. The Ministry of Education dictionary also cites chapter 24 of Romance of the Three Kingdoms, where an attack on an undefended Xuchang is proposed while Cao Cao is campaigning elsewhere.
These attestations should not be confused with a direct quotation from The Art of War. The idiom can now describe exploiting any lapse in protection, attention, or resistance, beyond a literal battlefield.
Sources and further reading
In everyday life
账号没有设置密码，骗子很容易乘虚而入。
Without a password on the account, scammers can easily exploit the opening.
Describes exploiting a weakness or gap in defenses.