I'll have to beg off

have to beg off

Said when one wishes to be excused from an obligation or invitation. She had to beg off that project when she realized that it interfered with her normal duties. They invited me to their dinner party last night, but I had to beg off because I was feeling ill.
See also: beg, have, off
Farlex Dictionary of Idioms.

I'll have to beg off.

Fig. a polite expression used to turn down an informal invitation. Andrew: Thank you for inviting me, but I'll have to beg off. I have a conflict. Henry: I'm sorry to hear that. Maybe some other time. Bill: Do you think you can come to the party? Bob: I'll have to beg off. I have another engagement. Bill: Maybe some other time.
See also: beg, have, off
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • beg off
  • appear to
  • a change of heart
  • a mystery to (one)
  • a shoulder to cry on
  • be badly off
  • a turn of phrase
  • able to do
  • able to do it
  • a tough customer/cookie