Mishegoss (pronounced mish-eh-GOSS) is a Yiddish word, borrowed liberally by that veritable Borg of a language, English as she is spoke. Especially by New Yorkers.
It's a wonderful catch-all of a word, but a the same time, it means something very specific. Hunting it down in written reference is pretty easy, but can be tricky, because both it and its root word aren't spelt using Western characters. They're Yiddish, as I said, which has been spelled different ways in different locales, borrowing writing systems to preserve a truth. It can be and has been written as mishegaas, meshugges and others.
So what does it mean?
Its base word is meshugge (or meschugge) - which means, roughly, 'crazy.' Those who are crazy are meshuggeners if they're guys, and meshuggenehs if they're girls. But one thing they all have in common is the mishegoss.
One Joyce Turner, of Cresskill, NJ, wrote to the New York Times in 1986 to explain the proper meaning of mishegoss. Later, in 2005 William Safire would have a more formal go at it in his famous NY Times column On Language.
BUT WHAT DOES IT MEAN, CUSTY, YOU FARKAKTE SCHLEMIEL???!
Sorry. Basically, it's the noun form, and means 'craziness.' Used in context, it usually refers to a particular eccentricity or visual indication of craziness exhibited by a meshuggener or meshuggeneh.
It has been used to great effect by great men.
(For those who don't know that quote, it is the incomparable Walter Matthau as Lt. Garber in the original 1974 version of The Taking of Pelham One Two Three.)