T.V.
Does she watch much TV? Some shows (even kids shows) have some language in them that you don't necessarily want your kids using. Does she go to daycare or hang out with any other kids (family or otherwsise?) That could be another place she picked it up. When she said it, did she seem to have any indication of a true "mean spirit" or do you think she was just repeating something she had heard but didnt truly understand the connotation? I know my daughter is 4 but even going back a while she will sometimes repeat some things I'd rather her not but I can usually trace it back to tv (the movie Shrek for example....)