In Australia, this term is used to describe a person who is silly,erratic, or lacking in sense. It is akin to calling someone an idiot, or a goose. Galahs are an Australian cockatoo that have a characteristically erratic and twisting flight which commonly makes them road hazards.
Someone who is mean and controlling could be called a tyrant
You are silly
It means they are a bully
Being called a goose would be like being called a little silly.
if you mean if someone does not believe in God, this person is called an atheist
It depends on the situation. Say you just won a gymnastics thingy. It was a long time ago I forget what theyre called. Someone says,"You are a cat" There it would mean your talented, your nimble.
When someone calls you a demon it means that you are evil.
If someone is called sadistic, it generally means that they get pleasure from seeing someone in pain or causing them pain. These days it is freely used when someone is being unnecessarily mean.
You could, but it depends what you mean. If you mean the person is a horrible beast, then you might want to say "Jabberwock" instead. The poem itself is called "Jabberwocky," so you would only be calling someone a poem.
If by "flattened" you mean "like someone took a sphere and sat on it", that would be an oblate spheroid.
It means that they are in love with you.
That you would set on fire for someone