Unicycle - Anyone that drives to make an escape on a unicycle is asking for certain death. Rollerskates are a much better option.
Univited - Anyone that turns up at your door univited is either a demon, a ghost of a previous owner, or a salesman. You should take a shotgun to any of them.
Unearth - Don't unearth things in a horror movie. It's very bad.
Undead - There are two types of undead: Vampires and zombie. There are many debates to which is cooler, but after the Twilight movies and True Blood, it was decided that zombies are the best.
Unknown - We should not fear the unknown. We should fear things that we KNOW want to kill us.
Ultra-absorbant Tampons - A great way to staunch blood flow, or avoid embarassing incidents with the female apocalypse survivors in your group. They can also be used to make ear rings if this is absolutely necessary.