Wyvern folklore is rarely fire-breathing like the four-legged dragon. Sometimes modern fantasy book and media authors have wyverns that are frost-breathing or poison-breathing instead of fire-breathing. The current spelling "wyvern" is not attested before the seventeenth century as "winged two-footed dragon". According to the Oxford English Dictionary, it is an alteration of Middle English "wyver" (attested thirteenth century), from Anglo-French "wivre" (cf. French "guivre" and "vouivre"), which originate from Latin "vīpera", meaning "viper", "adder", or "asp". Conversely, medievalist William Sayers proposes a more complex origin for the term. He notes that the Anglo-French "guivre" and its Middle English derivative ceased to
Westland Wyvern with smoke from a fuel leak onto the exhaust ducting. Clyde development was subsequently cancelled by Rolls-Royce after only 50 hours of flight time for the TF.2, and the aircraft was delivered to Napier & Son to be fitted with the Nomad turbo-compound engine. The latter engine never materialised, however and this aircraft was used for crash barrier trials. The first Python-powered TF.2 flew on 22 March 1949 and this aircraft introduced the ejection seat to the Wyvern. Twenty TF.2s were completed to the Python design although after three years of testing what was then a revolutionary aircraft design, a