To feel somewhat neutral about something. Not too heavy nor light.
It can also be a state of being; to be one to
go with the flow, to not care too much nor little.
Lightweight and heavyweight can be used
respectively.
"How did you feel
about that movie?"
"Eh, mediumweight. It was alright."
"Is he going to say anything about me
drinking the last of the coke?"
"Nah, he's pretty medweight
about that stuff."