Man, I gotta get me some of that tat....

Aside from the hopes of many adolescent boys, "tit" originally referred to something small, and came to be associated with small birds, which, in turn, came to be called "tits," such as the titmouse and the bushtit.

American Bushtits (Psaltriparus minimus) 

 "Tat," on the other hand, means anything cheap or poorly made.  So "tit for tat" means you're giving something small for something cheap, which is a fair trade, more or less – you're getting what you paid for.  Seen a slightly different way, tit for tat means equivalent retaliation.  If somebody whacks you, you whack them back.