That's a nice one! I recently found one very similar myself. Based on the plastic grip, that would place it around WWII. However, it's quite possible it's older than that. Are there any markings, no matter how small on the metal strip of the handle on the top and/or bottom. Also, look again at the flat part of the blade closest to the handle. Look carefully. Are there any worn marks or initials?
looks like a bayonet for the M1 Garand, modified by shortening it and removing the barrel loop. That is not the original sheath for it either. Looks like someone turned this one into a hunting /camping knife
Yup, that is an M-1 bayonet, someone did a lot of grinding on it. It's most likely like jewelryguy suggested, someone turned it into a hunting/camping knife. Strange its not marked, there should be a maker and an arsenal mark unless all of that was ground off too, or unless it's one of the modern Chinese copies that someone cut up.
There are some out there that were cut down during the war.... guys wanted a fighting knife and the bayonet was just too long for effective hand to hand combat. They're typically referred to a "theater made fighting knives" and if there was any way to prove yours was one of those it would have some collector value. As it is though it's just a cut up bayonet. Still a nice knife to have though.