A hex (with one x) can stand for a couple of things. The first is the idea of a curse or spell that is placed on an individual or family by a witch. This can explain the lyrics of Pavement's "The Hexx".
The song carries on the theme of Terror Twilight of an impending doom. As stated many times in this here blog, the last Pavement record served notice that the band's run had come to an end, but that's getting redundant.
Malkmus describes various situations in which his subjects suffer from hexes of their own. Capistrano swallows that are supposed to return to their San Juan home, suddenly can't. Epileptic surgeons and fooled football players prepare as their failures are close at hand. One of the greatest Malk-metaphors demonstrates this point further.
Architecture students are like virgins with an itch they cannot scratch,The 70's atmospherics support the gloom and doom of a hexed life. Bluesy guitar solos and wailing in the background set the mood of the track. One can picture a movie scene in which the protagonist is stumbling through a crowded hallway, intoxicated on some mind-altering substance, and dazed by his own failures as this music plays, drowning out the chaos around.
Never build a building till you're 50; what kind of life is that?
The other meaning to hex comes from a hexadecimal or a base-16 numbering system. Coincidentally, the song arguably has sixteen lines: five verses of two-lines each and a six-line chorus. Again, it's probably just a coincidence.