S.B.D.

From the delightful tome Depraved and Insulting English:

feist: A silent fart.

Thank you, Peter Novobatzky and Ammon Shea. I knew there had to be a word for it.

However, there's also this, from dictionary.com:

feist: a nervous belligerent little mongrel dog

The combination bodes ill for her: http://www.listentofeist.com/