不假思索

bùjiǎsīsuǒ
(idiom) to act without taking time to think; to react instantly; to fire from the hip

CEDICT

Entry Composition

1 不 bù
  1. no; not so
  2. (bound form) not; un-
=2 假 jiǎ
  1. fake
  2. false
  3. artificial
  4. to borrow
  5. if
  6. suppose
=2 假 jiǎ
  1. variant of 假 jiǎ
  2. to borrow
3 思索 sīsuǒ
  1. to think deeply
  2. to ponder