Synonyms for Impossible:

adj.

absurd (adjective)
foolish, invalid, preposterous, irrational, nonsensical, implausible, contradictory, incongruous, ludicrous, eccentric, senseless, farcical, silly, meaningless, illogical, inconsistent, anachronistic, fallacious, inane, bizarre, ridiculous, idiotic, paradoxical, crazy, outlandish, absurd, self-contradictory.
all (adjective)
unacceptable.
beyond the bounds of possibility (adjective)
impracticable, unobtainable, inaccessible, hopeless, unworkable, insurmountable, futile, preposterous, unfeasible, unattainable, impractical, infeasible, unthinkable, unimaginable, useless, unachievable, no-go, absurd, out of the question, inconceivable, impassable.
hopeless (adjective)
hopeless, unachievable.
impossible (adjective)
unattainable, absurd, impracticable, implausible, unfeasible, unobtainable, inconceivable, unworkable.
intolerable, ungovernable (adjective)
ludicrous, undesirable, improper, unsuitable, objectionable, offensive, incongruous, unacceptable.
mind-boggling (adjective)
unimaginable.
probable (adjective)
likely.
ridiculous (adjective)
farcical.
unbelievable (adjective)
incredible.

n.

difficult, delicate, critical, thorny, intractable, awkward, trying, improbable, unlikely, testing. hardly, unbelievable, no-go, something will not work/start/open etc., offensive, unsuitable, possibly, objectionable, with the best will in the world, be out of the question, undesirable, impossibility, improper. irritable, unmanageable, obstinate. impractical. insupportable, unbearable, unendurable, unsupportable, pain. balky, wayward, contrary, attitude, perverse, froward, support, contrarious, ornery. impossible (noun)
intolerable, unattainable, hopeless, unacceptable, unachievable, unworkable, out of the question, undoable, unfeasible, impracticable, out, unimaginable, infeasible, inconceivable, unrealizable, insufferable, unsufferable.

Other synonyms:

impossibility, unsupportable, ornery, contrarious. intractable, insupportable, froward, unbelievable, thorny, balky, unendurable, testing, wayward, unbearable. contrary, inaccessible. critical, possibly, perverse. delicate, difficult. discouraging
hopeless.
insuperable
insurmountable.
unrealistic
impractical.
Other relevant words:
delicate, unacceptable, critical, futile, insupportable, impossibility, unmanageable, unrealizable, hopeless, out of the question, likely, ornery, insufferable, untenable, undesirable, unbelievable, perverse, unlikely, unbearable, awkward, insurmountable, thorny, out, wayward, obstinate, offensive, unsupportable, unachievable, vain, undoable, impractical, froward, difficult, trying, unendurable, balky, unimaginable, possibly, unsuitable, objectionable, no-go, incredible, intolerable, improbable, infeasible, inaccessible, impassable, unsufferable, contrary, useless, improper, unthinkable, intractable, contrarious.

Usage examples for impossible

  1. I did not want to leave Ladysmith, but even if I had wanted, it would have been impossible – London to Ladysmith via Pretoria by Winston Spencer Churchill
  2. As soon as I could reason clearly I knew it was quite- quite impossible – The Mistress of Bonaventure by Harold Bindloss
  3. What the people ask for is impossible – The Social Cancer A Complete English Version of Noli Me Tangere by José Rizal