The 'Dark Tower' actor has been poking fun at his co-star after he was named Rear of the Year last week, but after giving him the moniker he checked to make sure it wasn't a worldwide accolade.
Matthew quipped: "He's got a trophy - the ass of class. "Is this an English award? Is it an international ass?"
In a joint interview with 'Entertainment Tonight', Idris replied: "It's not international, it's England. I think I'm Rear of the Year in England.
"That morning when the announcement came out, I took a good look in the mirror and I was like, 'Ok, fair enough.' "
Matthew saw the funny side of Idris' comments about his derriere accolade, but was left speechless.
He added: "I don't know ... I don't know what to say about that. I wasn't expecting [it]."
Tony Edwards, organiser and CEO of the Rear Of The Year Awards, admitted last week the 'Luther' star had won the prize for having a "well-toned, firm" behind.
He said: "Idris is also a kick-boxer which, along with his other workouts, has ensured a particularly well-toned, firm rear."
Tony - who awarded British TV presenter Rachel Riley with the female Rear of the Year gong - also insisted it is not always the "biggest butt" which triumphs.
He added: "Both Idris and Rachel's celebrity rears represent perfect examples of today's toned and firm gluteus maximus - with quality rather than quantity as the essential fashion keynote.
"Biggest is not best when it comes to bottoms."