I didn't think of anything else, so I thought I'd solve a mystery. We all know how someone who's a pill acts, but not why they are called a pill. Hello, Google search!
The most reasonable answer I could find is that pill is short for the British slang word, pillock. A pillock is apparently Scandinavian for penis. The Brits use it to refer to any stupid or annoying person. So, not so different really from our not-so-nice American word for someone acting like a jerk. Makes me think I won't be referring to anyone as a pill any more.