Synonyms of the word

undoable

• adj. Not doable; impossible to do.
• n. Something that cannot be done; an impossible or impractical task.
• adj. Possible to undo; reversible.

impossible

• adj. Not possible; not able to be done or happen.
• adj. (colloquial, of a person) Very difficult to deal with.
• adj. (mathematics, dated) imaginary.
• n. (obsolete) an impossibility.

unachievable

• adj. That cannot be achieved (or only with great difficulty).

unattainable

• adj. Impossible to attain or reach; inaccessible, unobtainable or unapproachable; unable to accomplish; beyond…
• n. Anything that cannot be attained.

unrealizable

• adj. Not realizable; unable to be achieved or realized.