Chuck Liddell -- I Was Happy to Let Randy Couture Get in My Shorts

UFC legend Chuck Liddell tells TMZ Sports he was HAPPY to fork over a pair of his trunks to Randy Couture for Randy's Halloween costume -- but says he ain't exactly returning the costume honor.

Liddell was out in Calabasas Monday when he told us Randy went through a celebrity middleman to get his hands on his trunks ... and Chuck obliged because he thought it was funny.

As for Chuck, two mysteries remain -- who will he dress up as on All Hallows' Eve ... and why is his right arm is in a medical brace???

Check out the clip.