Mush is a friendly term for mate, pal or friend - as in the friendly greeting, 'hello mush!'(rhymes with bush). It was common parlance in the south of England when I was a child, and I once read that it was an old Hampshire word. On searching the internet, I find that it's a Romany word meaning man, and 'Oi mush' should be taken as meaning 'excuse me sir'.

Mush is also the cry of dog-sled drivers to make their dogs move forward, derived from the French verb 'marcher' meaning to walk. It can also be the noun or the verb referring to the walk across the snow

http://www.picosearch.com/cgi-bin/ts.pl
http://www.dict.org/bin/Dict