Assuming you have some Galaxy phone. My previous phone was a Note 3 and it had that. There are 2 ways to disable it. The first is to long-press the back button, but when you reboot it will come back. The second is to turn off Multi Window in the quick panel in the pull down settings.
You rock. Thank you so much. Damn thing was driving me crazy cause no matter where I moved it, I would still always wind up accidentally hitting it. I thought it was a launcher issue. Didn't know it was my S5.