Soapy made a £10 bet with another VD'er over something or other, turns out that the other VD'er won the bet but Soapy never coughed up.
A Soapy Tenner is a £10 note that you will never actually see but still gives a modicum of weight to your argument by it's very presence.
From the example given we can see that even though the chance of the tomorrow being sunny is quite high, there is a possibility that it won't. You should therefore NOT risk your real tenner, but use your Soapy Tenner which has the built in option of non-payment.
The origin of the Soapy Tenner is a regular forum user at www.visordown.com you would regularly bet £10 on the outcome of various discussion and never pay.