IMPOSSIBLE

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"