I just found a musical source for the usually at least mildly insulting term "Bertha Butt".

This referent is used by African Americans {and maybe other folks} for women with very big butts. I don't know any comparable term for males with big butts unless it's Baby Huey, an insulting referent that comes from a cartoon character who was born large and lacked street wits. However, males who were called "Baby Huey" were big all over. The butts of males who were called Baby Hueys weren't the focus of derision {and a certain degree of lasciviousness} as is/was the case with women who are/were called "Bertha Butts".

The alliterative name "Bertha Butt" may predate the mid 1970s Jimmy Castor Bunch records, but I haven't found an earlier source-yet.

I'll post the lyrics and a YouTube video for the "Bertha Butt Boogie" in my next post to this thread.