Not to be confused with yellow-bellied, this is a slang term for someone who comes from Lincolnshire, England. In my dad's generation (born 1950), anyway.
As I understand it, there are two theories as to why this term arose. One is that the Lincolnshire regiment wore yellow tunics. The other is that the fields in Lincolnshire often contain yellow-flowered crops, such as mustard and rapeseed. The farm labourers would therefore get yellow petals and pollen on their clothes.
There is also a beer (bottled ale, not lager) named Yellowbelly.