A non-existant ten pound note, which is used for the purpose of online wagers or bets with others; where there is no intention of money changing hands regardless of the outcome.
The phrase originates from the online UK motorbike forum 'Visordown', specifically a user called 'Soapy' known for making bets with no intention paying anything.
The act of becoming so belligerently drunk that you pass out in the middle of walking, skipping, tap dancing, or cartwheeling, and thereby tumble on to the ground, or nearest solid object, in a topsy-turvy, yet charming fashion
"Oh my gosh! That drunk girl doing cartwheels accidentally tumbled onto that chain link fence!"
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 willnever 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.