Green’s Dictionary of Slang

(US) a general intensifier; a euph. for darned adj., damned adj.

1885		C.F. Lummis letter 5 Jan. in Byrkit Letters from the Southwest (1989) 228: ‘What a jesusly time’ he had killing off enough Navajos to keep the rest humble.
1946		J. Monaghan Tom Horn: Last of the Bad Men (1997) 93: [...] release Lieutenant Maus and do it ‘by-Jesusly quick’ or see their five friends shot down in cold blood.
2005		F. Mowat A Whale for the Killing 142: ‘Bust a sheer pin,’ one of them bragged, ‘but we cut a Jesusly big hole into her!’.