Weight weenies was originally a pejorative term for people who got too obsessed by the weight of their bike. They were considered to spend more time worrying about the weight of their bike rather than riding their bike. They would pay thousands of dollars to get a bike a few grams lighter, even if it sacrificed comfort.
The weight weenie for many cyclists was a person with more money than sense. Though now by using the weight weenies forum you’ll find that people who care about the weight of their bike have been retaking the term. Many though still spend way more money than they should in order to have a light bike.