出其不意

chūqíbùyì
to do sth when least expected (idiom); to catch sb off guard

CEDICT

to take somebody by surprise; to catch somebody unawares

Wiktionary

趁人不備，出於對方意料之外。《孫子．計》：「攻其無備，出其不意。」《三國演義．第五六回》：「等主公出城勞軍，乘勢拏下，殺入城來，攻其無備，出其不意也。」也作「乘其不意」。

重編國語辭典

出乎對方意料之外

維基詞典

Entry Composition

=1 出 chū
  1. to go out; to come out
  2. to arise; to occur
  3. to produce; to yield
  4. to go beyond; to exceed
  5. (used after a verb to indicate an outward direction or a positive result)
  6. classifier for dramas, plays, operas etc
=1 出 chū variant of 出 chū (classifier for plays or chapters of classical novels)
2 其 qí
  1. his
  2. her
  3. its
  4. their
  5. that
  6. such
  7. it (refers to sth preceding it)
3 不意 bùyì
  1. unexpectedly
  2. unawareness
  3. unpreparedness

Containing this entry

出其不意，攻其不備 chūqíbùyì,gōngqíbùbèi to catch an enemy off guard with a surprise attack (idiom, from Sunzi's "The Art of War" 孫子兵法 Sūnzǐ Bīngfǎ)