Merriam-Webster's Word of the Day for September 24, 2008 is:

bilious • \BILL-yus\  • adjective
1 a : of or relating to bileb : marked by or suffering from liver dysfunction and especially excessive secretion of bile *2 : of or indicative of a peevish ill-natured disposition 3 : sickeningly unpleasant

Example sentence:
Molly's bilious demeanor made her ill-suited for a job in customer service, and she was let go from the position after two weeks.

Did you know?
"Bilious" is one of several words whose origins trace to the old belief that four bodily humors (black bile, yellow bile, phlegm, and blood) control temperament. Just like "phlegmatic" ("of a slow and stolid phlegm-driven character"), "melancholy" ("experiencing dejection associated with black bile"), and the recent Word of the Day "sanguine" ("of a cheerful, blood-based disposition"), "bilious" suggests a personality associated with an excess of one of the humors -- in this case, yellow bile. "Bilious," which first appeared in English in the mid-1500s, derives from the Middle French "bilieux," which in turn traces to "bilis," Latin for "bile." In the past, "bile" was also called "choler," which gives us "choleric," a synonym of "bilious."

*Indicates the sense illustrated in the example sentence.