1) As a vocabulary item ‘wormstall’ is defined in the OED as ‘an outdoor shelter for cattle in warm weather’, and the inference is that ‘shelter’ refers to a building, such as a shed.
The first evidence quoted is in Philemon Holland’s translation of Pliny’s Natural History, dated 1601: ‘Drive thy sheepe and cattail out of the Sunne, into some worme-stalle and place of shade’. Wright defined ‘wormstall’ as ‘an outdoor shed or shelter into which cattle retire to avoid flies in warm weather’, a meaning previously suggested by other writers. The only evidence for ‘wormstall’ that I have found is as a place-name, and it was not infrequent in Yorkshire. It is likely though to belong to a much earlier period in English history since Old English wyrmsteall occurs twice in charter boundaries. Many people would therefore have been familiar with ‘Wormstall’ as a place-name long before the word was used by Philemon Holland in 1601. In Methley, for example, a manorial survey of 1592 has an entry for two closes called Wormestalles and the same field name is recorded in the court rolls fifty years earlier: 1543 ‘the steiles outside a close called Wormstall’. A similar reference is found in a Dissolution valuation of lands in Selby which had belonged to the abbey: 1540 ij closes called the Wormestall Closes cont. ix acres. An even earlier reference occurs in an undated charter for Longwood near Huddersfield, probably executed in the first two decades of the thirteenth century: ‘fifteen pence which Thomas his brother ought to pay for four acres of land in Wrmstalhirst’
that is a 'hirst' or small wood in or close to a 'wormstall'. Wormstall Clough in Barkisland near by occurs in a title deed in 1665. The spellings in the examples just quoted seem to rule out the possibility that the prefix ‘worm’ developed as a form of ‘oumer, umber’, as has been suggested, so it would be more logical to take it at face value and link it with Old English wyrm. This had a wide variety of meanings and could be applied to animals as diverse as toads and hunting dogs, for example: 1514 grewhondis and ratches and other smale wormys, Moor Monckton. As a common place-name ‘Wormstall’ may originally have meant something like ‘worm - place’ or ‘unpleasant creature - place’. If that is so, it raises a question about how we interpret Holland’s ‘worme-stalle’ which has been taken to refer to a building. Perhaps it was simply reinforcing ‘place of shade’, extending its meaning to ‘secluded’ or ‘out-of-the-way’. Markham used the word to mean a ‘shelter’ or shed in 1613 and this definition is one that many writers seem to have relied on ever since. It may be a scholarly invention.