Nice find! It looks like a hip flask, although it's quite small for one of those unless you have a huge hand. I don't remember ever seeing anything quite like it before. There were also pill bottles that used to be shaped like that but they were much smaller.
Yes, I dug one of those many years ago in a river that was part of a dam disaster. Many homes were destroyed leaving behind many 1850, 60s artifacts. The disaster was in 1874 in Williamsburg, MA. The flask is round bottomed and thin so it can easily slide into your back pocket. An amber corker. Probably a $10-$15 dollar bottle due to its shape. If it had embossing would be worth more no doubt. Thanks for sharing!
I agree that it's a pocket whiskey flask, much like the Biningers, Whartons and Klines. Although it has a cork stopper now, it appears to have a ground lip and originally took a screw cap, probably of aluminum.