Tim McGraw
is one hot 45-year-old.

The country superstar shows off his ridiculous physique on "The Ellen DeGeneres Show" today -- after a fan sent in a photo he took with Tim while out on a run.

And while we can't keep our eyes off those abs, Ellen zeroed in on something else: His Ellen boxers.

While he wears them with pride, check out the clip above to see why he was going commando during the interview!

Tim also opened up about his new sober lifestyle.

"My life has been a series of events that told me I shouldn't drink anymore, but I felt like it was time to grow up a little bit for me," he says.

"When it gets to the point where you think it’s affecting you adversely and it’s affecting your relationships adversely, it’s time to make a change and I thought it was," adds tim. "I also thought my girls are getting older and I certainly wanted to have some good solid ground to stand on when I started talking to them about their situations."

Good for Tim!

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