Green’s Dictionary of Slang

stuffing n.1

[late 19C+] (orig. US) the essence, the ‘daylights’, as in phrs. below; also used fig.

In phrases

beat the stuffing out of (v.) (also beat the stuffings out of)

[late 19C+] to beat up thoroughly.

knock the stuffing out of (v.) (also kick the stuffing out of)

[late 19C+] to beat up thoroughly, to defeat comprehensively.