"đi" here means go
"mà" here is actually a shorten version of "để mà..." (means to do something)
Vietnamese don't usually use this kind of phrase, this is from a poem/song and that's why it seems strange
This should be moved to idioms'...as you can't figure out the meaning based on just the literal translation of the words. Google says this means "wake up!" and it also says "đậy và đi' is 'get up and go".