Green’s Dictionary of Slang

(US) with great force, with absolute commitment.

1843		W. Oliver Eight Months in Illinois 93: But if it hadn’t been that I was the worse for liquor [...] I’d have whipped him like forty.
1856		Yorkville Enquirer (SC) 14 Feb. 4/3: ‘He’d been stayin’ at the Saint Charleses, an’ puttin’ it through like forty’.
1872		Schele De Vere Americanisms 313: Boys say, ‘You have scared me like forty,’ and teamsters boast of a powerful horse, that will pull like forty.
1875		M. Thompson Hoosier Mosaics 101: Shakin’ like forty — an’t ye, Zach?
1882		Dly Cairo Bulletin (IL) 1 Dec. 1/4: She was ‘sawing gourds’ like forty.
1899		St Louis Post-Dispatch (MO) 30 Apr. 50/5: I remember when my tooth was decade [sic] it aked like forty.
1925		Z.N. Hurston Spunk (1995) 953: A big black bob-cat [...] walked round and round that house and howled like forty.
1936		J. Held Jr San Diego Union 27 Dec. n.p.: Horns, confetti, streamers, rattles Making noises like forty battles!
1946		in G. Legman Limerick (1953) 1946: She fucks in the gloaming like forty.