The story behind the American pronunciation, Jag-wire, actually can trace it's roots to the early 19th century when the early American's were exploring the jungles of central and South America. Back in that period the adventurers would still use the Old English pronunciation of Jag-you-are.
However, in 1802 a British-American expedition to explore for Spanish lost treasure deep in Columbia would change all that. Unknowingly to the adventurers, a massive gaggle of Jaguars stealthily approached the rear of the expedition group. The beasts flanked on either side using their squirrel-like prowess to jump from tree to tree while they formulated an attack using telekinesis. At the last possible moment, little British-born Henry O'Finnigan, the expedition cook, recognized and tried to call out the impending attack to the others. He screamed, "Oi, you limeys dontchasee dis 'ere broodin', bloody, buncha Jag-you-ars." But it was too late, little Henry did not alert the leaders in time. Only two people made it back to civilization after that, the largest ever recorded Jaguar attack.
During the debriefing it was decided that the attack could have been wholly prevented had it not taken the farking Englishman so long to say a single word. Thus President Roosevelt signed a bill that stated, "From here forth Jaguar shall only only be pronounced Jag-wire, for the sake of time, ease of conversation, and in remembrance of the Columbia expedition. And as for the great empire England we're 1-0, neener, neener, neener, kiss my big, white, crippled, American ass." Thus launching a democratic whirlwind to change all the outdated Old English words into more friendly, easier to say words.