This phrase is used when someone does something wrong. You want to explain that the action doesn't fit what you hope or expect. You say "that wasn't very ___ of ___". For example, when a person does something mean:

That wasn't very nice of her.

Or when you do something that's stupid:

It's not very smart of me to leave my wallet sitting out on the table like that.

You often put a tag question on the end of this phrase, like "is it?" or "was it?"

That wasn't very nice of her, was it?

It's not very smart of me to leave my wallet sitting out on the table like that, is it?