ETYM Latin improbabilis; pref. im- not + probabilis probable: cf. French improbable. Related to Probable.
1. Having a probability to low to inspire belief; SYN. unbelievable, unconvincing, unlikely.
2. Not likely to be true or to occur or to have occurred; SYN. unlikely.
3. Too improbable to admit of belief; SYN. marvelous, marvellous, tall.