Adjective:
impossibbli, improbabbli, redikoli, inkonċepibbli, inverosimili, groteske, mhux mistennija, aneddottali, inkonsistenti, mhux xierqa, inkompatibbli, irrazzjonali, inkoerenti, inkomprensibbli, mhux palpabbli, impenetrabbli.
IMPROBABLE.
...whatever remains, however IMPROBABLE, must be the truth."
I believe what you meant to say is, "I give up. He's IMPROBABLE."
Well, if we accept your premise and also accept the highly IMPROBABLE assumption...