The Chiefs tight end's big bro slipped the comment in during the latest episode of "New Heights" ... while talking about the massive party everyone had following Kansas City's Super Bowl win on Sunday in Las Vegas.

Travis and Jason were laughing about how the elder Kelce lost his mind after discovering a Chiefs-colored luchador mask on a dance floor ... before they started talking about how hysterical it looked on him while they were at XS Nightclub.

Travis brought up one scene from the party in particular ... when he was with Swift on the side of the stage while Jason was in the DJ booth with Marshmello, waving his arms around like a mad man.

Travis couldn't stop giggling about it -- and that's when Jason dropped the L-word into the convo.

"On this side," Jason said of the now viral video of the situation, "two people in love and enjoying the moment together -- panning to a complete Neanderthal who is no longer connected with modern-day society."

Travis didn't remotely attempt to deny any of it ... chuckling through it all.

Of course, Jason is confirming what we had already known ... as hot mics caught Travis telling Taylor following the AFC Championship Game last month, "I love you."