Backstabber noun
Someone who says or does something harmful to you behind your back
There must be a backstabber in our organisation who has revealed our secrets to our enemy.
I would have tolerated her even if she turned into a backstabber.
He played backstabber to me when he talked bad about me behind my back.
1. Doing something extremely well
2. Behaving in an extreme or remarkable manner
1. Holly was in rare form during her speech, so she got the first prize in the competition last night.
2. Corrine: I'm going to buy Christmas presents for everybody. Do you want to go with me?
Laura: Oh...My kids are in rare form today, so I cannot go with you. Sorry!