I get a lot of questions about bourbon at work. In the hopes of saving myself some effort, I am putting some answers to common questions up here on the blog. That way, if you have been referred to here from my work, you can get a more in-depth answer than I could likely give you at the bar.
WHAT DOES PROOF MEAN?
Alternatively, why does this bottle say "proof"?
Proof is pretty simple -- figure out how much alcohol, by overall volume, is in the spirit, and double that number. You found its proof! (Basic algebra tells us we can reverse that too -- half of proof is ABV.)
The term proof came from how they used to measure alcohol -- they would mix a gunpowder with a bottle of booze and then set it on fire. If it burst into a bright flame (usually blue!) it would be "Proof" that the alcohol was good -- at least 50% ABV. Nowadays we have much more accurate ways of telling the amount of alcohol in a spirit (proof back then was actually a bit higher than the aimed-for 50%), but that was where the term came from, and it mostly just stuck.
Bourbon, to be bourbon, must be at least 80 proof, or 40% ABV. That means that in addition to containing weird flavoured additives that keep them from being real whiskey, a lot of "flavoured" and "spiced" whiskies are under 80 proof, further disqualifying them from true whiskeyhood. They're not my cup of tea, but if you like 'em, though, more power to you.