(via Quora) Answered by Tom Peracchio
When I first heard the song back in the 1970s I thought they were singing about some guy named Bruce. For years I sang along screaming “Don’t bring me down… Bruce.”
As ELO’s song writer Jeff Lynne explained below in an interview, it was simply a made up word, “Grooss.” Because so many people started singing it as “Bruce” he often just went with the common thought and sang it as Bruce when doing it live…