Dig In One's Heels Meaning

Example : 1938 July 16, Jean Kelvin, "The Tug of War," Glasgow Herald, p. 8 (retrieved 18 July 2011):  What we want are more women of combined business efficiency and integrity to get into public life and dig in their heels against the forces of war, lust, and injustice.1989 June 28, Flora Lewis, "Foreign Affairs: Maggie on the Beach," New York Times (retrieved 18 July 2011):  Margaret Thatcher tried to do it again, digging in her heels, lecturing archly on her achievements, illuminating our European partners on the superior virtue of her ways.2011 Jan. 27, Andrew J. Rotherham, "Fixing Teacher Tenure Without a Pass-Fail Grade," Time:  [T]he teachers' unions are still pretty much digging in their heels on the tenure issue.