Green’s Dictionary of Slang

a general intensifer, exceedingly, very.

1830		J. Galt Lawrie Todd 120: My wife, she's as the gentleman knows an almighty ambitious woman.
1834		Marryat Peter Simple (1911) 328: An almighty pretty French privateer lying in St. Pierre’s.
1846		Durivage & Burnham Stray Subjects (1848) 109: And when the news was brought to me, / I felt almighty blue.
1857		H.C. Kimball Journal of Discourses V 87/2: If you went into your field and you found an almighty big mustard stalk [...].
1904		Lewiston Eve. Teller (ID) 23 Sept. 4/1: We shall crowd an almighty big lot of business into this one week.
1912		J. London Smoke Bellew Pt 10 🌐 You’re almighty swift with business up here on the hill.
1919		Brier Wkly Times (MO) 23 Oct. 2/2: Though the wound may heal, it’s goin’ to leave an almighty big scar.
1961		C. Himes Pinktoes (1989) 167: Some people present who’re trying to be so almighty virtuous.
1967		J. Schaefer Mavericks (1968) 58: Them mules [...] They’re almighty tired.
2001		S. King Dreamcatcher 618: Several crows – there were some almighty big ones [...] had found the bread.