Generally refers to subjects of the male persuasion:
Someone who you dislike enough that you mustn't say their name out loud. And ironically, a way to call a friend that makes you laugh and is odd enough to warrant the title.
A deceptive, stealthy, yet lazy person.
A sleeping large pig.
That sombysow has slept with every girl around here, not to mention he owes me 20 bucks!