An inflammatory term for someone who is acting weird; i.e. freakish, gross, annoying or overreacting to little things.
Also used as a generic insult with no real meaning other than to offend.
Used in a multitude of forms.
"Look at Andrew over there trying to flirt, what a grebe."
"Who's the fucking grebe who stole my wallet?"
"Dude, he doesn't want to stab us, he doesn't even know us. Stop grebin' out."