The same as shakin' in my boots, which means scared. The phrase is used sarcastically often.
The specific term comes from Austin Powers in which Dr. Evil says "I'm shaking in my little space boots", although some falsely attribute it to Toy Story. Also, the "little" is commonly skipped from the phrase.
Someone who is addicted to obtaining money and building wealth. A money addict and fanatic. Breadheads often work more than one full-time job, and some even participate in illicit activities to "obtain the bread".