A yellowbelly (or yeller belly) is a person from Lincolnshire, England. The origin of this nickname is disputed, and many explanations have been offered. These include:
The officers of the Royal North Lincolnshire Militia used to wear bright yellow waistcoats
The uniforms of the old Lincolnshire Regiment were green with yellow facings. The fastenings of the uniform tunic, which were known as frogs, were also yellow.
A species of newt, frog or eel (there is disagreement on this point) found in the Lincolnshire Fens had yellow undersides.
It is a derogatory name, implying that the Fen-dwellers creep around in the mud, and so get yellow bellies.
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.
Sheep grazing in mustard fields were dusted by pollen from the blossom that turned their undersides yellow. Alternatively, the long under wool of sheep grazing in the Lincolnshire Wolds became discoloured by the yellow clay. (Neither of these can be considered a unique occurrence in Lincolnshire however)
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.
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.
The stage coaches that operated in Lincolnshire in times past had yellow body work.
A folk etymology says that 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.
Less reputable inhabitants of Cleethorpes may have engaged in a form of piracy. They took torches down to the beach on dark nights and used them to trick passing ships into sailing towards the lights believing it to be the harbour. The ships would hit the shallow waters on the Cleethorpes coast and run aground where they would be raided for their cargo. On one occasion the locals are supposed to have believed they had lured a ship whose cargo had great value, but when they found the cargo was actually a vast amount of yellow flannel material. They could not be seen selling or using the cloth they had stolen so in order not to waste it they made undergarments out of it, so many of the locals were soon in possession of flannel vests and became yellow bellies.