fantastic(also fantastical), implausible, incredible, incredulous, unbelievable, uncompelling, unconceivable, unconvincing, unimaginable, unthinkable

debatable, disputable, dodgy(chiefly British), doubtable, doubtful, dubious, dubitable, far-fetched, fishy, flimsy, questionable, shaky, suspect, suspicious, unlikely, unreasonablehopeless, impossibleabsurd, comical, farcical, laughable, ludicrous, outlandish, preposterous, ridiculous, risible, sillyindefensible, insupportable, untenable

full of it

believable, cogitable, conceivable, convincing, credible, creditable, imaginable, plausible, supposable, thinkable

likely, possible, probablereasonablecertain, incontestable, indisputable, indubitable, questionless, sure, undeniable, undoubted, unquestionable