Both tit and tat are archaic words meaning 'a light blow'. The entire expression thus means 'a blow for a blow', like 'an eye for an eye'. Both words were used as verbs, too: a popular song of the late sixteenth century had a refrain, "Come tit me, come tat me,/Come throw a kiss at me."

These words are probably of imitative origin, with a vowel variation found in other words expressing striking such as tip and tap or pit-a-pat. The tit is not related to other tit words, such as the ones (each of independent origin) meaning 'a small bird' (e.g. "titmouse"), 'a breast', or the first element of "tit-bit" (in America usually euphemized to "tid-bit," but not related to the 'breast' word).

The phrase tit for tat is first found in the sixteenth century. It is probably a variant of tip for tap, of similar origin but found a century earlier. The tip in this earlier phrase is the same word as in the baseball expression "a foul tip."

"If a day comes when the world of Islam is duly equipped with the arms Israel has in possession, the strategy of colonialism would face a stalemate because application of an atomic bomb would not leave any thing in Israel but the same thing would just produce damages in the Muslim world"
Ayatollah Ali Akbar Hashemi-Rafsanjani December 2001