Used by Spartans to demonstrate to Persian messengers the first, second, and third laws of motion. Upon kicking the messenger over the ledge, the messenger exerted an equal force back upon the Spartan. At the same time, gravity accelerated him down the seemingly endless hole, and he will continue moving with a high velocity until he encounters the ground and transfers momentum to the ground, equal to his acceleration times his mass.
In reality, the phrase is simply one of the most awesome phrases ever uttered before.
Used in forums or gaming to signify ownage.
You're playing call of duty 4
and kill four people at once with a .50 caliber sniper rifle.
You: "THIS IS SPARTA!!!"
Others: *running away*
You're on a forum and a noob is spamming.
You "THIS IS SPARTA!!!"Noob
Noob gets banned.