What is I Don't Believe It!?
1.
phrase (var. I do not believe it!)
an expression of utter disbelief at a truly unfathomable circumstance. Popularised by the grumpy old man Victor Meldrew (Richard Wilson) from the popular BBC television situation comedy series,
From the sitcom;
On discovering a lamppost had crashed through his bedroom window 'Victor Meldrew' exclaimed "I don't believe it!"
Colloquial use;
Dave: Look Barbara, malt loaf is on 'buy one get one free'
Barbara: I don't believe it!
See