A Leather Cookie is a person who lives their life tanning out in the sun all year round, to the point of literally looking like their skin is made of leather. They are usually very flabby, wrinkly, dark brown, and flat when they're laying on their floral collapsible chair, and simply cook themselves in the heat of the sun. Therefore, they perfectly resemble a thoughtless cookie. Unfortunately they do not have the common courtesy to cover their shiny, leathery, gelatinous flesh with more than a bikini... to, of course, attract even more attention to their 'gorgeous' (and time consuming) complexion, and can cause blindness. If you live in Florida, however, it can be very contagious. These people are indefinitely prone to skin cancer.
"What's that dark splotch laying on the sand over there?"
"I don't know dude, looks like a beached whale or something."
"Ooohhh no it's just a leather cookie!"
"Ew, dude, how is that thing still alive?"
"It's probably got a week or so until skin cancer finally gets to it..."