From the beginning of the film 300 when reject babies are tossed off the cliff as they don't meet the rigorous Spartan standard.
Can be used as a general insult or specifically in relation to twats that think they are Spartans.
If someone says SPARTA! loudly, implying that they are a Spartan warrior or just generally trying to big themselves up then you reply...
Shut it cliff baby
Your mamma tossed you from the cliff