qiāo​dīng​zuān​jiǎo

to make doubly sure

  1. to knock
    to hit, strike, or tap; to beat a drum; to rap on a door or surface
  2. to nail
    to drive a nail into something; to join or fix things in place using nails or staples
  3. to drill
    to drill or bore a hole by rotating a pointed tool
  4. base
    bottom or base of an object, such as a wall, mountain, or piece of furniture