t i t For Tat

What's the meaning and origin of the phrase:
t i t For Tat?

The original was Tip for Tap, a very old expression that evolved into t i t for tat in the mid-16th century. Tip meant a hit, a shove. Tap was a retaliatory hit. The meaning is a punishment meted out to pay back an offence, measured to be equal and proportionate. You hit me, I hit you, t i t for tat. By the way, t i t for tat turns out to be the optimal strategy in Game Theory to guarantee cooperation from an opponent.