Green’s Dictionary of Slang

(usu. Aus./N.Z.) a general intensifier.

1931		L. Merryweather ‘Hell in Amer. Speech’ AS VI:6 434: He seems to be in a hellishin’ hurry about something. This is a hellishin’ fine time to tell us about it.
1941		Baker Popular Dict. Aus. Sl. 35: Hellishun, an intensive used widely by children.
1991		W.T. Vollmann Whores for Gloria 101: She pulled down the straps of her top to show her hellishly beautiful breasts.