Green’s Dictionary of Slang

to beat severely.

1914		Wash. Times (DC) 20 Oct. 5/2: What would she be doin’ [...] but fall [...] in love with Paddywhack himself, the same as what used to paddywhack her proper when she was a naughty little girl.
1950		‘Hal Ellson’ Tomboy (1952) 59: Afraid of getting paddywhacked?
1954		J. Steinbeck Sweet Thursday (1955) 76: Sometime when I ain’t feeling yellow I’ll paddywhack you.