E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A commercial term meaning that the article referred to is more than its nominal value. Thus, if you must give more than £100 for a £100 share in a bank company, a railway share, or other stock, we say the stock is above par.
If, on the other hand, a nominal £100 worth can be bought for less than £100, we say the stock is below par.
Figuratively, a person in low spirits or ill health says he is below par.