Green’s Dictionary of Slang

(orig. US) the very best, superlative; thus the popular description of Ella Maxwell (1883–1963), ‘the hostess with the mostest’; also as adj.

[1859		Punch 31 Dec. 263/1: I’m to say it’s all his own compojink and that the hother pair of dustmin was a couple of low blaggards and he ave left his card to show as he’s the mostest proper one].
1885		Indianapolis Journal 15 Nov. 10/4: We set around the kitchen fire an’ has the mostest fun a-list’nin' to the witch tales ’at Annie tells about.
1889		Century Mag. Oct. 960/1: ’Nd lookin’ like a reg’lar queen — Th' mostest like I ever seen.
1947		Billboard 14 June 161: [advt] You can still see FIESTA at your distributors—it's the ‘mostest of the bestest for the leastest’ .
1956		‘Toney Betts’ Across the Board 16: I worked for Dan Parker, ‘de mostest’ sports columnist! He has the mostest readers.
1959		M. Richler Apprenticeship of Duddy Kravitz (1964) 270: Your mag is the mostest!
1960		C. Cooper Jr Scene (1996) 70: It was the mostest the most could be.
1979		‘Iceberg Slim’ Airtight Willie and Me 165: Brandy was the supreme hard-on enticer with the mostest.
2023		R. Milward Man-Eating Typewriter 121: What angered me mostest about the petty scab was his unflinching willingness to degrade himself for extra dinarli .