He does not specify this unprovable, complex, unplausible, and useless hypothesis.
In this he is the very opposite of the austere Scot who sits midway of the table; he cannot tell a lie in an unplausible way.
But something happened—something unforeseen and unexpected and unplausible happened.
1540s, "acceptable, agreeable," from Latin plausibilis "deserving applause, acceptable," from plaus-, past participle stem of plaudere "to applaud" (see plaudit). Meaning "having the appearance of truth" is recorded from 1560s. Related: Plausibly.