To become attracted to or sexually aroused by intelligence and its use.

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.

Where the Dunning-Kruger effect and the bedroom collide. An alleged sexual orientation for people whose assessment of their own intelligence is far in excess of that of any reasonably objective observer, with the added benefit of making one a soi-disant minority, presumably an oppressed one.

People who describe themselves as "sapiosexual" usually define "intelligent" as "sharing my biases" rather than, you know, "intelligent". Given the first paragraph, this invariably means they're attracted to people as arrogant as them, who they run off with and have a clutch of horrible, spoilt children with who grow up into horrible narcissistic adults.

Furthermore, all it means is they're attracted to people similar to them, making the term fairly tautological to begin with.