These are your hints:
1) Asness is currently a Marvel comic book collector, in addition to his duties at AQR Capital
2) As we know from Scott Patterson's The Quants, as a teenager in on Long Island, Asness was "obsessed with little besides girls and comic books"
3) Also from The Quants, the following image: "Friday morning at AQR, August 10. Asness glanced pensively at a candy-colored array of Marvel superhero figurines lined up along his east-facing window. Spiderman. Captain America. The Hulk. Iron Man."
4) In a new profile of Cliff out tomorrow in Bloomberg Markets magazine, the manager sits on his desk flanked by said dolls, in a full-page glossy photo.
Okay, give it your best shot.
If you answered "a shield of Captain America," that's right! Richard Teitelbaum, who wrote the Bloomberg story called "The Quant Who Won't Shut Up," discusses said tattoo in a podcast found here, shamefully admitting to not snapping a pic of the goods.