Dooms ‧ day /ˈduːmzdeɪ/ BrE AmE noun [uncountable]
1 . till/until Doomsday informal for a very long time:
You could wait till Doomsday and he’d never show up.
2 . the last day of the Earth’s existence, when everything will be destroyed, according to some religions
doomsday scenario (=a description of a very bad and hopeless situation)