What's the meaning of the phrase 'Beyond belief'?
Outside the range of what is normally considered believable.
What's the origin of the phrase 'Beyond belief'?
It was Shakespeare who first used the word 'beyond' in a general 'outside the range of' sense, as opposed to its original 'at a great distance' meaning. This usage is found in Julius Caesar, 1601:
These things are beyond all use.
In The Tempest, 1610, he also extended usage to the 'unbelievable' meaning that is the 'beyond' of 'beyond belief':
Which is indeed almost beyond credit.
None of Shakespeare's works includes the expression 'beyond belief'; for that we have to wait a few years. John Gower's 1616 translation of the Latin poem Festivalls by Ovid (Publius Ovidius Naso) includes the first example of 'beyond belief' that I can find in print:
Nor could I dream that death purgu'd her grief.
Ah me! her courage was beyond belief.