E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To find a mares nest is to make what you suppose to be a great discovery, but which turns out to be all moonshine.
Why dost thou laugh?
What mares nest hast thou found?
Beaumont and Fletcher: Bonduca, v. 2.
Are we to believe that the governor, executive council, the officers, and merchants have been finding mares nests only?The Times.
N.B. In some parts of Scotland they use instead a skates nest. In Gloucestershire a long-winded ta≶e is called a Horse-nest. In Cornwall they say You have found a wees nest, and are laughing over the eggs. In Devon, nonsense is called a blind mares nest. Holinshed calls a gallows a fouls nest (iii.). In French the corresponding phrase is Nid de lapin; Nid dune souris dans loreille dun chat. (See CHAT.)