E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Stock at par means that it is to be bought at the price it represents. Thus, £100 stock in the 2 1/2 per cent. quoted at par would mean that it would require £100 to invest in this stock; if quoted at £105, it would be £5 above par; if at £95, it would be £5 below par. (Latin, par, equal.)