It stands for neurotypical.
What neurotypical means depends on the context:
1) person who is not on the autism spectrum
2) person who has no significant neuro-psychological conditions defined in the DSM, including "mental illnesses," learning disabilities, etc.
3) The square, boring, stupid, common, and possibly mythical "normal people" that every subculture on the Internet likes to think they're better than.
_________________
Right planet, wrong country: possibly PLI as a child, Dxed ADD as a teen, naturalized citizen of neurotypicality as an adult