Where did the term "Welshing on a bet" or "Welching on a bet" come from?

Answer 1

Here's an explanation that makes some sense:

"English bookmakers to avoid paying out on large bets sometimes ran over the border into Wales, and consequently were said to have gone Welsh."

Answer 2

if i thought about it, in my opinion, think about this one before giving me a thumbs down. everybody expected the underdogs to lose, as they should, theyre the underdogs, the pats were undefeated. It is probably known as an upset because, like everyone thought and said "the patriots are going to demolish them, 36-6" as an example, everybody was expected such a team to just destroy the giants like they destroyed every other team, but it was upsetting to see the best team, 18-0, the team everybody thought would win, the best team, to lose. that is an upset, not a shock. it was upsetting, and im sure many people lost much money from it. that is upsetting, and i guess you could say its a shock too, it was a shocking upset, nonetheless.