generally "extremely good"
common in UK during
mid-70s when it was most often applied to
rock music, in which context it would also imply "strong" or "
powerful"
the opposite was helluva tame, generally but not necessarily used in a derogatory context.
Barker: "what kind of sounds do you dig, man?
Tender:
"Oh, Zeppelin, Sabbath, Heep .... any hellish beef stuff really"
Barker:
"
Yeah me too, Zep are the men - I even like their helluva tame stuff on Zep 3"
Tender: "Wheyyy.... look at that
guy with the hellish beef prime Plant locks over there!"
Barker: "
Yeah, he looks beef but he's carrying a helluva tame Cat Stevens album under his arm"