improbable
1. [ adjective ] not likely to be true or to occur or to have occurred
Synonyms: unlikely
Examples:

"legislation on the question is highly unlikely" "an improbable event"

Related terms: probable supposed
2. [ adjective ] having a probability to low to inspire belief
Synonyms: unconvincing unlikely unbelievable
Related terms: implausible
3. [ adjective ] too improbable to admit of belief
Synonyms: marvelous tall marvellous
Examples:

"a tall story"

Related terms: incredible
Similar spelling:   improvable
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  impregnable
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  impressible