A surety is a fact that no one questions. Surety, in a legal sense, is the security you can provide when you apply for a loan.
When you're sure of something, you're positive it's true. Such things are sureties, because they inspire such confidence. Also, there's a legal type of surety. If you take out a loan, a house you own could be a surety: it could be claimed by your creditors if you don't repay the loan. That type of surety provides security (or insecurity, if you fail to pay back the loan).