below par

At a price below the face, or nominal, value of a security, especially a bond. Generally, the difference between the price paid and the amount received upon sale or maturity is taxed as a capital gain.

Also, under par. Not up to the average, normal, or desired standard. For example, I am feeling below par today, but I'm sure I'll recover by tomorrow. This term employs par in the sense of "an average amount or quality," a usage dating from the late 1700s.