No, I don’t mean someone who pilfers wine.
A wine thief is a glass tool for sampling wine from a barrel.
Some small wineries here will let you sample their wine directly from a barrel, and many will use a wine thief to get the wine. Yes, this exposes the rest of the wine in the barrel to oxygen. So this should only be done if the barrel is soon topped off, or a layer of CO2 is added to the barrel immediately to protect the remaining wine from oxidizing.
My wine thief is of a traditional design, but with a modern feature as it has a glass stopper at the end to prevent wine from spilling out.
Simply by depressing the exposed tip of the stopper in a wine glass the wine will flow out of the thief and into the glass.
I am all for tradition. But I find the glass stopper method much better than the traditional way of preventing wine from flowing out of the end of the thief. The traditional method was to plug one end or another with a finger to either preserve the suction (plugging at the top) or to stop gravity flow (plugging at the bottom). In Hungary small cellars often will plug a wine thief at the bottom from which the wine flows out. Because that is traditional and plugging at the top is harder to set and control with the thief design often used here (note: there are modern wine thief shapes that are easier to plug at the top — but they look like a big pipette one would expect to find more in a chemical lab than in an old world winery).
The problem I have with the finger at the bottom method is I really start to wonder where has that finger been lately, and when was the last time that person washed their hands?