Sorry about that! I hope that you weren't eating!
This is called the brief safe. You stash your valuables in the crotch of a "dirty" pair of men's undies. It's brilliant and gross all rolled into one but I bet that it would work. They are for sale on Amazon for $12.99 but I'm betting that we could make our own. I have also heard of stashing your valuables in a diaper at the beach. Have any other ideas on how to keep your stuff safe on a trip? Stash everything in a tampon box?