Jack Osbourne is ripping trolls a new one after they started shaming him over his weight loss -- insisting he's not "grossly underweight" ... as they apparently dragged him for just shaving his beard and getting a "creepy mustache."

In a new TikTok clip, Jack said the backlash is insane after he spent years getting mocked for being overweight -- adding he hasn't lost any additional weight since competing on the UK reality series, "I'm a Celebrity ... Get Me Out Of Here!" 6 months ago.

Jack explained that he decided to overhaul his lifestyle after hitting 220 pounds around three years ago ... dropping down to 155 pounds at 5'8" -- which he says is perfectly healthy.

He sealed the rant with a furious "get f***ed" to the haters ... while fans rushed to his defense.

Looks like body-shamers just won't get off the Osbournes' backs -- with Sharon and Kelly constantly catching heat over their appearances ... as the fam's clearly had enough of people policing their bodies!