1. Short for this for that.
2. To scratch each other's backs.
3. To trade services.

Maryann helps here friend Becky with Becky's homework.
Becky is now obligated to help Maryann with something she needs under the "tit for tat" rule.

Anatidaephobia is defined as a pervasive, irrational fear that one is being watched by a duck. The anatidaephobic individual fears that no matter where they are or what they are doing, a duck watches.

Anatidaephobia is derived from the Greek word "anatidae", meaning ducks, geese or swans and "phobos" meaning fear.

To show one's tits in exchange for seeing another's tats

A special day of the year when tattoo parlors will give free tattoos to women if they flash their tits.

A term made famous in a Jane Russell movie where she stands large- breasted between two cowboys. One cowboy says "You stole my horse so I stole your girl." The other cowboy answers "Yeah, tit for tat."

The practice of a female’s either showing a man “The Merchandise” or letting him feel her up in exchange for his assistance. Often mentioned in the case of a normally-modest/celibate female who does not want to expose herself but feels that she may have no choice at the time because she really needs the male’s immediate help but possesses no money or other "acceptable" way to presume to expect said male to exert himself on her behalf.

When a woman's tit is seen accidentally by a man, it is only right that the man show his "package" to her as well.
And if you're lucky, it may lead to something else..

Also works vice versa.