Unbelievably rhymes with:
conceivably, irretrievably, conceivably, irretrievably, acceptably, admirably, affably, affordably, amiably, amicably, appreciably, arguably, audibly, bearably, charitably, comfortably, comparably, conceivably, considerably, consolably, credibly, creditably, demonstrably, discernably, enjoyably, equitably, fashionably, favorably, feasibly, flexibly, forcibly, formidably, honorably, horribly, impeccably, implausibly, impossibly, improbably, inadmissibly, inaudibly, incomparably, incredibly, indelibly, inescapably, inevitably, inexcusably, inexorably, inexplicably, inextricably, inflexibly, inseparably, intangibly, intelligibly, interchangeably, intolerably, invariably, irreparably, irresistibly, irresponsibly, irretrievably, irreversibly, irrevocably, justifiably, knowledgeably, measurably, memorably, miserably, notably, noticeably, ostensibly, palpably, passably, peaceably, personably, plausibly, possibly, predictably, preferably, presumably, probably, profitably, reasonably, recognizably, regrettably, reliably, remarkably, respectably, responsibly, sensibly, suitably, tangibly, terribly, unacceptably, unaccountably, unalterably, unavoidably, unbearably, uncomfortably, uncontrollably, undeniably, understandably, unequivocably, unfavorably, unjustifiably, unmistakably, unpredictably, unquestionably, unreasonably, unseasonably, unsustainably, variably, veritably, visibly
Unbelievably sounds like:
unavailable, unbelievable, unemployable, unflappable