The use of this expression by non-native Chilango speakers will make any native really proud. It is informal yet not offensive at all. Pura piña (POO-rah PEE-gnah) literally means "pure pineapple". It is used to refer to something fictitious, made up or simply a decoy. Pura piña is often preceded by la, as in la pura piña.
Colega: ¡No mames, Menchaca!, cómo le has chingado para obtener el puesto de gerente. Ya ni te veo.
Menchaca: Es pura piña. Me la paso tirando la hueva todo el día hasta bien entrada la noche.
Coworker: Holy shit, Menchaca! you've seriously worked your ass off to be promoted to manager. I don't see you anymore.
Menchaca: It's all fiction. I've been slacking all day until very late in the evening.