Wikipedia
Bittereinders were a faction of Boer guerrilla fighters, resisting the forces of the British Empire in the later stages of the Second Boer War (1899-1902).
By September 1900, the conventional forces of the South African Republic and the Orange Free State had been largely defeated by the British army. The remnants of Boer government resolved to fight on in a guerrilla war, to try to force the British to retreat from the territory. As it became clear that military victory was unlikely, opinion among the guerrillas divided between those who wanted to secure a negotiated peace and those who preferred to fight on to 'the bitter end'. The decision to fight was given particular currency by British use of concentration camps.
The word is derived from the Dutch language upon which the Afrikaans language of the Boers (later known as Afrikaners) is based. The words bitter einder (singular, the final "s" denotes plural) literally mean "bitter-ender" denoting someone who is "willing to fight to the bitter end".
Taken more generally, it could be used as another name for a "war party" (a faction within a political or military group favoring the waging of war) or for any group which does not wish to diminish its "fighting spirit" wanting to fight it out to the "bitter end."