Why Does ELO's "Don't Bring Me Down" Say Bruce?

Ultimate Classic Rock has cracked the code on that "Bruce" line in Electric Light Orchestra's "Don't Bring Me Down."  Turns out the line isn't even "Bruce" but rather a vocal ad-lib made by Jeff Lynne saying "groose."

Lynne said he only kept the "groose" in after his German engineer asked him "how did you know that word?  Gruss.  It means 'greetings' in German."  Lynne decided "That's good.  I'll leave it in." and the rest is misheard lyrics history.