It was also popular in the late 50s.
The Urban Dictionary quotes the following:
https://www.urbandictionary.com/define.php?term=Mush
Not sure about the geography reference.
Over the years I have been on the receiving end of the following: Mush (in my teen years, an expression that seems to have died out), Duck, Love (by Northern shop workers), Boy (as an adult by an older well respected colleague), Mate, Mon (by a Scot) and Mun (in Wales). I have not been offended...