Green’s Dictionary of Slang

1. (also heckashin, hellashin, hellashus) wonderful, amazing, extraordinary.

1929		in Botkin Folk-Say I 53: It was a hellashus fight.
1934		H. Sebastian ‘Negro Sl. in Lincoln University’ in AS IX:4 289: hellacious Outstanding.
1943		M. Constiner ‘The Turkey Buzzard Blues’ in Ruhm Hard-Boiled Detective (1977) 272: You got a hellacious style if you don’t mind my sayin’ so.
c.1943		S .J. Baker Aus. Vulgarisms [t/s] 5: Hell: hades, Halifax, heck, hellishun, hangashun, help (and, possibly, heavens).
1965		B. Adler Vietnam Letters (2003) 21 Aug. 101: There did ensue the most HELLACIOUS downpour ever witnessed by civilized man!
1975		G. Underwood ‘Razorback Sl.’ in AS L 1/2 61: We had a hellacious time at the Rink Saturday.
1978		‘A.P. Gaskell’ All Part of the Game 158: It’s hellashin funny. [Ibid.] 191: The more acceptable variations were ‘hangashin’ and ‘heckashin’.
1983		‘Heat Moon’ Blue Highways 235: That first jump off Pitt must have been hellacious.
1999		J. Ridley Everybody Smokes in Hell 71: She was a hell of a woman. Particularly hellacious was her skin.
2004		(con. 1960s-70s) T. Taylor Top Fellas 57/2: The site of much hellacious brawling.
2006		F.X. Toole Pound for Pound 66: Chicky [...] was hellacious as the best defensive back on the high school football team.
2021		J. Ellroy Widespread Panic 50: Mattress Jack [Kennedy] was a hellacious hophead.

2. difficult, demanding.

1967–8		Baker et al. CUSS 136: Hellaceous [sic] Difficult exam.
1987		C. Hiaasen Double Whammy (1990) 66: We had one hellacious busy morning.
1997		Simon & Burns Corner (1998) 309: It’s hellacious work, with the midday sun beating down and the heat radiating up from the gray-blue stone and creosote tiles of the railbed.