1
general::
Phrase(s): below par
not as good as average or normal. • I feel a little below par today. I think I am getting a cold. • His work is below par, and he is paid too much money.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms