"O sir, sir, sir!"
The son, son, son! of Collier's folio is much better. She never elsewhere says Sir to him. See on Meas. for Meas. iii. 1.
"The thwartings of your dispositions."
'Thwartings' is the emendation of Theobald for things of the folio.
"I have a heart as little apt as yours,
But yet a brain," etc.
There is, I think, either an aposiopesis at the end of the first line; or a line is lost, as Volumnia is speaking quite calmly; or, to stoop, to yield, or something of that sort is omitted.