A person of substandard intelligence (usually an emmit) who enters a particular chain of stores with the aim of annoying staff by upping footfall counts unnecessarily; This is usually followed by a complaint about something they wouldn't have bought even if they could afford it.
Less commonly used to describe people entering a store just to hide from what ever kind of weather happened to be prevailing at that time.
E.g. Walking into a shop that you have no interest in being in, then back out to fetch the wife, then back in again to show her a garment that you don't like so that you can have a good whine about it and/or its price.
Usually followed by a member of staff saying "bloody showerporker, that's ruined our conversion rate"