The word seems to be an adaptation of 'firewall', a term deriving from the 1970s indicating a system that blocks access to a computer to protect it from security threats. That in turn derived from the name of a physical partition designed to inhibit the spread of fire in a building. In recent years, paywalls have gone in and out of favour, as publishers struggle to make money on the web.
The newspaper world is currently mesmerised by the progress of The Times's website, placed behind a paywall in July. But commentators also now refer to Sky TV acquiring more and more content from the free-to-air channels as using a paywall strategy. If anyone can, Rupert Murdoch can make a paywall pay well.