usually "not very good", "weak", "lacking in substance"
Commonly used in the UK during mid 70s as a disparaging adjective. However, occasionally could be used to describe rock music in a less negative manner, in which context it can also imply "gentle" or "sensitive"
The opposite was generally hellish beef
Barker: "what kind of sounds do you dig, man?
"Oh, Zeppelin, Sabbath, Heep .... any hellish beef stuff really"
"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"