stands for "mental health day"
when you're too tired/lazy to go to work/school, so you call in sick. basically a euphemism for faking sick.
Mom: Billy, time to get up for school!
Billy: But I am le tired.
Mom: Then have a nap.
Billy: Can I take an MHD?
Mom: Fine, you lazy piece of shit.
a blind, often aggressive belief in what one's textbook says, in violation of the facts. Derived from the military term "combat snobbery."
French kid: My textbook said capitalism is evil!
Smart kid: And that's why those who live in socialist countries are so poor and unhappy?
French kid: ... but it said Americans are evil!
Smart kid: You seem to suffer from textbook snobbery, sir.