The term etard is a shortened version of the slang term retard using the somewhat standard Internet notation of using the leading "e" to indicate electronic, as in E-mail. There is no polite way to say it, basically, an etard is a stupid user. Many consider that etards are largely responsable for most of the ills of the Internet because if etards did not respond to phishing, did not download things they should not, did not click on links when they don't know where they go, did not respond to spam at all then all those bad things would go away from lack of interest. I doubt that's completely true but as with everything there likely is a large grain of truth in the sentiment.