Brett Favre used to be great, now he's just a big jamook.(n)
I don't know why I dated him, it was so jamooky. (adj)
Get your shit together, don't jamook on me. (v)

additional uses:

jalook: lovable jamook- someone who is clumsy or anti-loquacious, but altogether endearing nonetheless. ex: elephants, toddlers, student teachers

jamick: dick jamook- the worst breed of jamook, this person's jamookness is obnoxious, discourteous, often barring any possibility of another individual being attracted to this person. ex: chinese drivers, dps office workers, Hitler.

jalick: lovable dick jamook- a jamook who often acts irrationally and impolitely, but you love them nonetheless, or in some cases you love them because of their jamick qualities. ex: Simon Cowell, Joe Flacco, Amy Winehouse.