In order to understand this phrase, think about what is going on in the previous scene. In that scene, Rosalind has fallen completely in love with Orlando. She is overcome with the strength of her emotions. She also seems to suspect that bad things are going to come of her love, probably because she realizes that Duke Frederick is not at all happy with Orlando.
Because of this, she is not really herself. She just doesn't want to talk to anyone. This is what she is saying in the line you cite. Celia has asked her if she does not have any words. She replies that she does not have even a word to throw at a dog. She is just saying in a very hyperbolic way that she does not feel like saying even one word.