>>2856313A boot knife is any knife you carry in/on your boot. There are reasons that you may want to do this but frankly if you're asking /out/ about it then you probably don't have a practical reason. Soldiers in certain roles might have gear the makes belt carry impractical, you often see (or at least saw) that in airborne troops. That's also why Chris Redfield carries his knife the way he does in RE1. I carried a knife in my boot in high school because I was often accosted by homeless people on the way home but it was only in my boot at school for concealment, it was in my pocket as soon as I was on my way home so I could actually get to it.
You're probably determined to do this no matter what anyone tells you so I recommend that you get the S&W HRT, it's a copy of the Gerber Mk1. Everyone in China makes a copy of the Mk1 but S&W's comes with a better sheath than most and even if you use this thing most of what you'll be doing is carrying it. If you're willing to spend a little more money then Cold Steel's integral dagger is nice and also comes with a nice sheath.