big lie

Short definition from Wiktionary

Noun

1. 1
   The policy or practice of insistently making a false claim which is so emphatic and grandiose that listeners and readers will reckon that the claim must be true because no one would dare to fabricate something so forceful and extravagant; a false claim produced by the application of this policy or practice.
2. 2
   Alternative letter-case form of big lie.