If the IRS looks into it at all, they might want to know how you came to have $100,000 in cash. If it was taxable cash income that you did not report on your tax return, they will charge you the back taxes plus interest and penalties. Depending on the circumstances, they might also dig deeper to see if you had any other income that you did not report. You might need a lawyer. If you might be on the hook for a large amount of tax, you might want to hold onto some of the money so you can use it to pay the tax, instead of giving it all away.
If you and your wife filed a joint tax return for a year that you had income that you didn't report, she is equally responsible for the back tax, interest, and penalties.
If you live in a community property state, any income that you received while you were married is probably community income, which means that your wife is responsible for reporting her share of it, even if she filed separately. If the cash that you give your wife is from community income (whether or not the income was reported), then only half of it is a gift. The other half belonged to your wife all along.
In spite of all this, though, giving the money to your wife is a nontaxable gift and you do not report it.
