Derived from a combination of words used to describe the process of reverse peristalysis, or a forceful exit of vomit from the oral orifice.
Hork and Hurl, two commonly used words for vomit, have been combined to make the Hall portion of this phrase.
Blair finds itself derived from the pop culture reference of Linda Blair, star of "The Exorcist" and arguably it's most famous scene of projectile green vomiting.
One can also use this phrase in mixed company. If you are attempting to be discreet about your feelings, most laymen do not catch the vague reference.
ex. That creep was so skeezy I feel like Hall-Blairing right now. Let's jet.