A Bronx man suffered severe burns to his backside in a raging house fire yesterday because he had his pants sagging, sources said.

Rasheed Garcia, 22, had to jump from the third-floor window of the building on East 219th Street in Williamsbridge at about 10:45 a.m. to flee the flames that scorched his lower back.

“He had his pants around his buttocks. He got burned on the buttocks. If he had pants on, he wouldn’t have gotten burned,” a source said.

The blaze broke out in a first-floor bedroom where 52-year-old John McFadzean had been sleeping.

McFadzean suffered critical injuries, while Garcia was in stable condition.