Wiktionary
unprovably
adv. In a way that cannot be proven.
Usage examples of "unprovably".
But had Dondo believed his own rubbish, or had this been a clever plan to use Teidez, obliquely and unprovably, to disable or assassinate Orico?
Stephen's eyes had closed again, but this time in what Renie felt unprovably sure was something closer to normal sleep.