Green’s Dictionary of Slang

(US black) to beat a person to death, thus making them into an Egyptian-style mummified corpse.

[1792		Sporting Mag. Oct. I 15/1: I beat my last tagonist in a giffy, and made a mummy of him].
[1801		Sporting Mag. July XVIII 194/2: A young Lord has made his terrible to all the coachmen [...] in London [...] he having often, as the phrase is, beaten them to mummy.].
[1830		Egan Finish to the Adventures of Tom and Jerry (1889) 236: She was almost beaten to a mummy by the jealous rib of the tradesman].
1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].