Compounds of 恐々

• 恐々
  (adj-na,int,n) respect (epistolary usage)
• 恐々
  (adv) fearfully
  timidly
  cautiously
• 恐々謹言
  (exp) Very truly yours (a complimentary close used in a letter written in an old epistolary style)
• 戦々恐々
  (adj-na,adj-no,adj-t,adv-to) trembling with fear
  filled with trepidation