The ENQUIRER goes behind bars to get inside the unrepentant world of the HOT CAR DEATH DAD, Justin Ross Harris – the most hated father in America!

The Georgia monster who left his 22-month old son Cooper inside a sweltering car showed no remorse while being booked into county jail – and chatted with other inmates as if nothing at all had happened!

The revolting behavior was witnessed by Mark Wilson, 21, who sat next to Justin Harris, 33, for two hours in the Cobb County Jail booking room.

“There was no emotion whatsoever in his voice, attitude – anything,” the Rome, Ga., college student told The  ENQUIRER in an exclusive interview. “There was just no sadness in how he was acting.”

Mark was unaware of the death of little Cooper when Harris strolled in and greeted the more than 30 inmates as if he had run into pals at a tavern. “The first words out of his mouth were, ‘What’s up guys?’ as if nothing had happened.”

The dad’s upbeat demeanor was in sharp contrast to his dramatic, June 18 “What have I done!” meltdown after discovering he had left his son baking in his SUV.

Incredibly, while at the jailhouse processing center, Harris was so relaxed he took a 15-minute nap! Although Harris quickly became chummy with Mark, who’d been given a one-day sentence for DUI, he never revealed his alleged crime.

Eager to chat, Harris asked anyone within earshot, “So what are you in here for?” and demanded, “How long is this going to take?” When a film was interrupted for a jailhouse announcement, Harris muttered, “Now we can’t watch TV, I guess.”

It wasn’t until Mark arrived home the next morning that he read about Harris’ case.

“It was shocking,” he said. Harris remains in jail without bond and faces murder and child cruelty charges.

Added Mark, “I don’t see how a father in that situation could act completely normal just hours after it happened…I don’t think it was an accident!”