1. The nickname came about because of a Royal Warrant of 1751 confirming the colour facings of the marching regiments of Foot, the 10th had bright yellow, this lead to the Lincolns being given the nickname the yellowbellies, the regiment had fought for this Great County since 1685 as Grenville's regiment before becoming the Lincolns.
2. The traditional breed of sheep in the county was the Lincoln Longwool. These sheep would often graze in the fields of mustard that were once a common sight around Lincolnshire. As their shaggy coat dragged along the ground it would pick up pollen from the mustard flowers and give them, you've guessed it, a yellow belly.
The Lincoln Longwool.
4. Women traders on street markets in past times are reputed to have worn a leather apron with two pockets, one for copper and silver and one for gold. At the end of a good day they would say they had 'a yellow belly' meaning they had taken a large number of gold sovereigns.
5. The expression is based on the old belief that if a person born in Lincolnshire placed a shilling on their abdomen on retiring to bed and slept flat on their back all night, then the next morning the shilling would have turned into a gold sovereign.
6. Many a Lincolnshire family used to keep a pig, and after the killing the curing bacon sides turned yellow. Some reckoned that some locals ate so much bacon that they themselves turned yellow.
7. Lincolnshire was full of poachers, the pollen of meadow flowers was rubbed onto the poacher's belly as they crawled through the fields stalking their prey. The constabulary of the time regarded this as proof positive that the individual concerned had been up to no good and could well earn a trip to the colonies.
8. A farmer had a rather overweight daughter that no one wanted to marry so the farmer decided to use an incentive. To the person who would marry his daughter he would give as many gold coins as would cover her stomach as she lay on the ground. Hence 'yellow belly'.
9. Due to a festival celebrating pototoes when people would paint their bellies yellow to resemble a potato.
10. The traditional clothing for a farm worker many years ago was the smock made of light coloured canvas material. Apparently when this material was subjected to sunlight and the garment became older it turned a yellowy colour, hence "Lincolnshire yellow belly"
11. The women would gather the mustard from the fields in their aprons, their aprons would thus be coloured yellow.
12. The mail coach that ran from Lincoln to London had a yellow undercarriage. Upon it's arrival in London it is said that the locals would call out "Here comes the Lincolnshire yellowbelly".
13. A species of newt, frog or eel (there is disagreement on this point) found in the Lincolnshire Fens had yellow undersides.
14. Opium extracted from poppy heads, and taken to relieve malaria that was prevalent in the fens in earlier centuries, turned the skin a shade of yellow.
15. It is a derogatory name, implying that the Fen-dwellers creep around in the mud, and so get yellow bellies.
16. The term originated from Elloe, the name of the rural deanery that serves the fen area of the Lincoln Diocese. This in turn took its name from the Saxon Wapentake which was referred to as Ye Elloe Bellie - Elloe meaning out of the morass while bel was the Celtic word for hole or hollow.
Any more out there that I've missed?