You shouldn’t wear slippers barefoot if you don’t want to get athlete’s foot.
If you have a naked foot, it means you're not wearing anything on your foot, making you barefoot.
The reading here is a bit weird. It's like the words 裸 (はだか) and 足 (あし) were smushed together into one word. The 裸 becomes はだ and the 足 becomes し. You can think of it this way: You were barefoot too long, and because of this, you don't have feet anymore, though you had ashi (はだし) at one point in the past.