Armie Hammer -- Boxes Ex-NFL Badass ... Gets BLACK EYE

Armie Hammer thinks he's such a badass boxer ... he dared to step in the ring against a former NFL linebacker whose nickname was "Lights Out" -- which explains the shiner on Armie's eye.

TMZ Sports got this shot of Armie's bruised face after his sparring match with ex-Chargers star Shawne Merriman -- who's 6'4" and 280 lbs! Nonetheless, we're told Hammer's the one who sought out Merriman as a sparring partner.

The bout went down Wednesday at Unbreakable Performance -- a new gym on the Sunset Strip owned and run by ex-Bears star Brian Urlacher and Jay Glazer from FOX Sports. We're told Armie's been training with Glazer for several months ... and word is everyone in the gym was stoked Armie wanted to challenge Shawne.

We're guessing Armie's agent isn't so stoked about his swollen face -- which makes it even cooler he's doing it. Can't wait for the rematch!