Used to apply to any person or item that, although being successful in a particular field, is not the best there is. Especially relevant when there is a clear dominating person/product etc. in their field.
In the same way that in breakfast cereals, you can eat other cereals, but there's always a voice in the back of your mind saying "it tastes good etm, but it's not Coco Pops."

From STEM, an abbreviation short for "Science, Technology, Engineering and Mathematics", a STEMlord is a pejorative term for a particular kind of person who has studied in these areas at university or works in these fields who holds a pretentious, condescending attitude to anyone who has studied in any other fields, particularly the Arts and Humanities. They typically believe themselves to be more "intelligent" and "rational", while generally remaining incredibly ignorant outside of their areas of expertise and having the charisma of a rotting pig's backside.