Green’s Dictionary of Slang

1. unwell, emotionally low.

1943		W.R. Burnett Nobody Lives for Ever 126: I’m worrying about you. You haven’t been up to par since we got here.
1949		H. Miller Sexus (1969) 31: He could laugh readily enough, a good healthy laugh, too, but when he subsided he was always a bit below par.

2. less than successful.

1902

E. Townsend Chimmie Fadden and Mr Paul 23: ‘Not up to par,’ he says. ‘My queue got tangled in me niblick yesterday.’.

3. of less than average intelligence.

1920

Ade Hand-made Fables 81: This Traveller was a City Yap, which is worse than being a Begosher, because the R.F.D. Boob usually knows that he is below Par.