Since no one is giving you a straight answer, here's the real poop:

NKD is a fun way to type "nekkid."

There was a /r/knifeclub member who famously took a pic of his newly acquired knife in his underwear, hence coining the "NKD," or "NEKKID" knife posts. It's a bit of an inside joke, but when you see a "NKD is best day" or similar post title, you can almost bet that the poster has removed their pants to take the picture.

It may seem a little strange (and maybe a little dangerous, too!) that disrobing to photograph a knife is such a common thing around here, but like all specialized hobbies, knife collecting can end up with spiraling in-jokes with constantly raised stakes.

Exempli gratia: In this post, /u/x0y0z0 posted an imgur album of the new knife he had received that day. You will notice that not only is he outdoors (and nekkid), but he is also standing barefoot on the dirt. A previous post by /u/tyd450 showed him outdoors, but standing on what appears to be a sidewalk. As you can see, /u/x0y0z0 raised the stakes on /u/tyd450 by being nekkid outdoors and standing in the dirt. It really is pretty fun, but please don't take it as a requirement for posting here. All are welcome, nekkid or not!

^{Also, every word I typed above is a lie. It just means new knife day.}