乘虛而入

乘虚而入
chéngxūérrù
to enter by exploiting a weak spot (idiom); to take advantage of a lapse

CEDICT

趁著對方空虛不備的時候進入。宋．張君房《雲笈七籤．卷一二○．范陽盧蔚醮本命驗》：「將至所居，自後垣乘虛而入，徑及庭中。」《三國演義．第二四回》：「今曹操東征劉玄德，許昌空虛，若以義兵乘虛而入，上可以保天子，下可以救萬民。」也作「乘隙而入」。

重編國語辭典

Homographs

乘虛而入 chéngxū'érrù

Entry Composition

1 乘虛 chéngxū to take advantage of weakness
2 而 ér
  1. and
  2. as well as
  3. and so
  4. but (not)
  5. yet (not)
  6. (indicates causal relation)
  7. (indicates change of state)
  8. (indicates contrast)
3 入 rù
  1. to enter; to go into
  2. to join; to become a member of
  3. (bound form) to conform to (as in 入時 rùshí)
  4. abbr. for 入聲 rùshēng
  5. (on product packaging, after {number n} + {classifier}) containing (n pieces) (from Japanese 入 "iri")