Could either be described as
a) a one night stand type situation where you have sex and never talk to each other again
b) or it could just describe any situation where you just do it and leave ( for instance, partners can still mash and dash each other )
Usually used among people we describe in the UK as 'rudeboys'
Nah, blud, last night was just a mash and dash, you get me, she aint my girl or nuffin.