Redskins Fans -- TROLL COWBOYS FANS ... At Dallas Airport

Talk about pouring salt in the wound!!!

A Washington Redskins superfan grabbed a microphone at the DALLAS AIRPORT this morning and led the D.C.-bound crowd in a rendition of the team fight song ... and TMZ Sports has the footage.

The guy who grabbed the mic was in town for the Cowboys vs. Redskins game Monday night -- and was clearly still fired up about his team emerging victorious!

So, while waiting for his flight back to D.C., he grabbed a mic from the flight attendant's stand and asked all the Redskins fans who could hear his voice to join him in singing "Hail to the Redskins."

They did ... and it was LOUD!!