Aaron Hernandez -- DRAWING NAKED WOMEN ... to Pass the Time in Jail

Aaron Hernandez is like an incarcerated Peter Paul Rubens -- 'cause he's passing the time by drawing naked big-bootied ladies in jail ... and TMZ Sports has obtained one of his fat-bottomed originals.

Hernandez fired the letter off to a male penpal right before Thanksgiving ... and after going on about how he's still innocent, and still not sweating the murder charge yada, yada, yada ... he punctuated his letter with a sketch of a Kardashianesque nude.

And like a true, serious artist, he included a caption: "Dats a bad bitch lol."

Not bad work, but we're guessing the New Yorker won't be calling anytime soon.