Adjective. Of fiction and narrative, a story / character / etc with pretensions towards being epic, mythical and majestic but which falls short, usually ending up being pretentious, cluttered, noisy and / or bombastic instead.
Corruption of the adjective 'mythic', based on how it might be pronounced in certain accents and regional dialects (esp. South / Estuary regions of London).
Last night's episode of Doctor Who was trying so hard to be epic, but it just ended up being myffic instead.