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 (and equally troubled) Blackburn Firebrand. Several second-line squadrons also received Wyverns around this time. Total production was 127 airframes with 124 aircraft completed, as the last three Eagle piston-engined airframes, VR138, -139, and -140, were never completed. The first carrier trials were carried out by the first pre-production Wyvern TF.2 aboard on 21 June 1950. Despite this, when the Wyvern S.4 entered service with 813 Naval Air Squadron in May 1953, it had not obtained clearance for carrier operations, this being obtained only in April 1954. The Wyvern was in service with the Fleet Air Arm from 1954 to 1958.