Why is a woman's breast called a 'tit'?

The slang word tit derrives from the word teat, which is correctly pronounced either as tĭt (tit) or tēt (teet).

A teat is the small, exterior part of the body from which milk is discharged. For humans, the teats are the nipples on a woman's breasts; for cows, teats are the finger-shaped protrusions on the udder.

The slang word tit does not correlate directly to teat, since the term tit refers to the entire breast, not just the nipple.