'improbable' originates from Latin, specifically the word 'improbabilis,' where 'im-' meant 'not' and 'probabilis' meant 'provable or credible.'

'improbabilis' transformed into the French word 'improbable,' and eventually became the modern English word 'improbable' through Middle English.

Initially, it meant 'not provable or credible,' but over time it evolved into its current meaning of 'not likely to be true or to happen.'