I am Southampton born and bred and when I grew up in the 50`s and 60`s it was used both as a term of friendship - "Alright, Mush??" - (in fact my dad still uses it in that way) and in the same way as the word "bloke" i.e."these two Mushes were walking along the road", but as far as the origin of the word, I was told it was from the Romany meaning " man ".