whoever spelt "du" as "dou" is a dumbass
the phrase originates from south vietnam.. you might also here "dit me may" which means the exact same thing, but is used my nothern vietnamese people for their dialect is slightly different by means of accent and a few words.
DU MA MAY means fuck your mother..
DU MA means mother fucker..
"du ma thang sau nay" = "mother fucker, this ugly guy" or.. "this mother fucking ugly guy"