A dullard; a philistine; one whose witless bumbling has caused the destruction of something beautiful, such as keeping an artistic inspiration from becoming reality.

This is most commonly an unfair characterization, as it probably was for the now legendary person from Porlock who interrupted Samuel Taylor Coleridge while he was trying to transcribe the poem he had just dreamed, Kubla Khan.

Nevertheless, the term has crept into our vocabulary, as everyone has been through at least one experience where they could point to someone (perhaps themselves) and say "You really screwed the pooch, you Person from Porlock!".