Adjective impossible has 3 senses
- impossible - not capable of occurring or being accomplished or dealt with; "an impossible dream"; "an impossible situation"
Antonyms:  possible, accomplishable, achievable, doable, realizable, affirmable, assertable, allegeable, attainable, come-at-able, gettable, getable, attemptable, conceivable, contingent, feasible, executable, practicable, viable, workable, fermentable, mathematical, researchable
 - impossible, inconceivable, out of the question, unimaginable - totally unlikely
Antonym:  thinkable (indirect, via unthinkable)
 - impossible, insufferable, unacceptable, unsufferable - used of persons or their behavior; "impossible behavior"; "insufferable insolence"
Antonym:  tolerable (indirect, via intolerable)
 
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Noun impossible has 1 sense
- impossible - something that cannot be done; "his assignment verged on the impossible"
--1 is a kind of impossibility, impossible action
 
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