Definition of "embittered"

[…] they were opposed to it on grounds that were at once legal, political, historical, personal, geographical, and mixed; the whole combination of causes operating of necessity to make the opposition an embittered one.

1884, John William Wallace, An Old Philadelphian: Colonel William Bradford, the Patriot Printer of 1776. Sketches of His Life, page 141

Again, by means of the ‘follower’s’ life, it will be revealed who is the admirer; for the admirers will become highly embittered against the follower.

1944, Søren Kierkegaard, translated by Walter Lowrie, Training in Christianity, page 245

Dawkins’ book is, oddly, much more embittered and deeply emotional, although leavened by his effervescent wit.

2011, Philip W. Anderson, More and Different: Notes from a Thoughtful Curmudgeon, page 177