E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To sell the skin before you have caught the bear. To count of your chickens before they are hatched. In the South Sea mania (1720), dealing in bear-skins was a great stock-jobbing item, and thousands of skins were sold as mere time bargains. Shakespeare alludes to a similar practice:
The man that once did sell the lions skin
While the beast lived, was killed with hunting him.