Muckrunner and tortfeasor are currently my two favorite words (though one isn't really a word). Tortfeasor is a genuine and real word, muckrunner is a compound word derived from a misinterpretation of a homophone. So as to allow you to enjoy your expanded vocabularies, I present, without further ado, the definition and derivation of tortfeasor and muckrunner (respectively):
Tortfeasor : One who has committed a tort. For those that don't know, a tort is a civil wrong that is not a breach of contract or, in other words, those things that people sue each other for all the time. Thus, a tortfeasor is someone who has wronged someone else in a non-breach of contract sort of way. Mostly though, it just sounds really neat.
Muckrunner : Derived from the term running amok, which is sometimes written running amuck. Running amok is to act in a violent, murderous frenzy and can be considered roughly synonymous with going postal or killing spree. Based on running amuck sounding an awful lot like running a muck, I figured that one who runs a muck might be a muckrunner and thus the derivation is obtained. When, however, one drops the original meaning of running amok, a muckrunner can become a totally different thing entirely. Imagine, if you will, a man driving an old rusty pickup truck filled to the brim with some indescribable muck; now, further imagine this man driving down old dirt roads in a breakneck, Dukes of Hazzard style much like the prohibition era smugglers. Although muckrunner is an improper derivation of running amok, I find it to be a wonderfully amusing construct.