Well, it’s partly a Southernism, but a lot of Blacks, regardless of their location, tend to use it. It’s a deliberated exaggeration of “naked”, and “nekkid” implies folks up to no good (for example, one standing outside alone in the woods buck naked is just naked. One standing buck nekkid in front of one’s boyfriend or girlfriend means something is about to happen. ;-))
Oh, I get the nekkid part (I’m from the south) but the butt vs buck is what I find interesting. I grew up hearing buck, but more recently hear only butt.
susie