He's mean (4)

I believe the answer is:
norm
This might be a double definition.
'he's' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'mean' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for norm that I've seen before include "expected thing" , "Average level of performance" , "Average level of achievement" , "Mean" , "Small boy" .)
