My wife was owed 40k in back childsupport she never received it and now the statue of limitations has run out (washington state) and the amount is now 0 can we right this off as a loss on taxes?

No, as you are mistaken. If there is a court order in place then they have not run out. She is still owed the money. There is no such limit on child support owed.

First, this is not considered a taxable loss. Even if she is never paid the arrears she is owed, its not tax deductible as a loss. Second, as cdad said, there is no statute of limitation on a child support award. What is making you think there is?

Do not know about the statute of limitations issue, but the non-payment is definitely NOT tax deductible for two reasons.

1) You cannot claim as a deduction the non-receipt of income.

And

2) Child support is neither taxable income for the recipient nor tax deductible for the payor, so a tax deduction is not an issue in any event.