Trent Williams -- The 'Silverback' Gets a Silverback on His Back

Washington Redskins offensive lineman Trent Williams is 6'5" and weighs 318 pounds, so just imagine how much ink was used to slap a gigantic tattoo on his back.

WIlliams -- whose self-annointed nickname is "The Silverback" due to his aforementioned massive size -- got the tat a few months back of ... what else ... a silverback gorilla.

The ink cost Williams around $3,000 (plus a "nice tip") ... courtesy of Sergio Garcia at Ultimate Designz in Miami. We're told Williams had to sit through two separate seven-hour sessions ... which he took like a champ.

Hard to imagine a man that size crying.