In As You Like It, what does Rosalind mean when she says, "Not one to throw at a dog," to Celia?In Act 1st scene third

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.