An impossibility
Late 14th century: “Madame,” quod he, “this were an impossible!” — Geoffrey Chaucer, ‘The Franklin's Tale’, Canterbury Tales
Not possible; not able to be done or happen.
It is difficult, if not impossible, to memorize 20,000 consecutive numbers.
(of a person) Very difficult to deal with.
You never listen to a word I say – you're impossible!
Imaginary
impossible quantities, or imaginary numbers