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"