Definition of unbirthday in English:

unbirthday

noun

humorous
  • Any day except one's birthday.

    [as modifier] ‘an unbirthday present’
    • ‘I wanted to buy you an unbirthday cake as a surprise.’
    • ‘Having demonstrated to his own satisfaction, if not Alice's, that unbirthday presents are to be preferred because people can have them more often, he adds triumphantly, There's glory for you!’
    • ‘And, as Humpty Dumpty noted: ‘There are three hundred and sixty-four days when you might get unbirthday presents’.’
    • ‘They can draw money out of 401 plans and IRAs to celebrate their unbirthdays or prepare for their big 60th bashes.’
    • ‘And Mrs Du Faur may have been the first and last person to celebrate ‘unbirthdays’.’
    • ‘If you are under the age of, say, eight, you'll think this is your very own unbirthday present.’
    • ‘Ahhh, but there are three hundred and sixty four unbirthdays!’

Origin

1871: coined by Lewis Carroll in Through the Looking Glass.

Pronunciation:

unbirthday

/ˌənˈbərTHdā/