Sentence:  What I suffered with that rein for four long months in my lady's carriage, it would be hard to describe, but I am quite sure that, had it lasted much longer, either my health or my temper would have given way. Before that, I never knew what it was to <WSD>foam at the mouth</WSD>, but now the action of the sharp bit on my tongue and jaw, and the constrained position of my head and throat, always caused me to froth at the mouth more or less. 
Meaning of the word marked with <WSD> tags:  To spew saliva as foam.