It's inevitable. You could say a million and five words and the one your kid picks up on is the four-letter one. (And we're not talking about "work.") Now what?
Ignore. Ignore. Ignore. A lot of kids repeat curse words because they get a reaction. If you don't give that reaction, the interest in the naughty word will dwindle.
Trade up. Try to give your kid an equally "exciting" word or phrase to say in its place—something like "Oh my gosh!" or "Holy smokes!" Do you best to sell this new word as far more appealing than the original offender.
Don't stress if she swears in public. As much as you might want to hide under the table, consider this: when your baby or toddler says a swear word in public, chances are only you can understand it anyway. And if it does come out clear as day, realize that it's happened to every other parent in the room at least once.
If your child continues to insist on swearing like a trooper or engaging in a lot of "potty talk" (another delightful phase that tends to set in at around age 3), you'll have to find ways of letting her know that such language is unacceptable—perhaps sending her for a time-out to let her know that you don't want to hear any talk like that. (One mom sends children who want to engage in "potty talk" to the bathroom.)