Green’s Dictionary of Slang

a diehard, one who does not give up until the bitter end.

[1850		Speeches in Congress on admission of California 341/2: The disunionisl looks to a southern Confederacy; the bitter-ender to the triumph of his party].
1907		Dundee Courier 23 Nov. 4/2: The former stands for those who are best known as ‘bitter-enders’.
1917		Sunderland Dly Echo 21 Dec. 2/3: No one knows whether he is tame or savage — a pacifist or a bitter-ender.
1920		Dundee Eve. Teleg. 15 June 4/2: Mr Stanton’s stand as a ‘bitter-ender’ during the war was resented by a section of the electors.
1933		Nottingham Eve. Post 8 Feb. 8/4: Hard cash now available [...] is likely to look more attractive even to the bitterest ‘bitter-enders’.
1944		Sun. Post (Lanarks.) 3 Sept. 3/2: [headline] ‘Completely Mad’ Nazis are Bitter-Enders.
1982		G.V. Higgins Patriot Game (1985) 62: Digger was inviting the bitter-enders to leave.
2019		V.D. Hanson Case for Trump 314: Obama had good reason to anticipate a whiny, petulant, and bothersome post-election defeated Trump in exile. He was forewarning the country not to listen to such a likely bitter ender.