Tit

Tit, meaning breast, comes from Old English for teat.

Tit, meaning small,

like Titmouse

has a Viking origin and apparently has nothing to do with the theory that 12th century Scandinavian women has tiny breasts.

Tit for Tat
May have come from the phrase "tap for tat" meaning blow for blow. Better than the alternative - the only dictionary definition for tat is "the act of making lace" and the phrase "breasts for lace" implies that frilly bras were imagined long before they were invented.