Taffy was a Welshman, Taffy was a thief, Taffy came to England, and stole a leg of...

[Osweo]

Bore da, Welschers!

This may be of some interest:
http://www.theapricity.com/forum/showthread.php?t=2260

On the whole, I don't think it's very likely at all, and the more traditional border raiding explanation is spot on.

[Treffie]

Lol! I think it's only me and Cynan who are actually native Welsh, but thanks for that Mr Oswiu.

Ulf also sent me it in Plaut Deitsch.

http://www.pgs.org/dialect_audio.asp

Btw, it would be too far for me to run into England, steal some beef, run back home and cook it for my family!

[Osweo]

Btw, it would be too far for me to run into England, steal some beef, run back home and cook it for my family!

Aye, but I bet those poor long-suffering folk in Pembroke and the Gower's Englishries are not immune from your foul subhuman Beefokleptic tendencies...

[Treffie]

Aye, but I bet those poor long-suffering folk in Pembroke and the Gower's Englishries are not immune from your foul subhuman Beefokleptic tendencies...

You're well read Mr Oswiu!

[Osweo]

You're well read Mr Oswiu!

You better believe it. Not that you make it hard to find out, showing off about it in your literature;
It's all here.

Got it off Amazon.