Welsh valleys slang. To drive around in a car (usually a clapped-out Escort/Nova covered in cheap spoilers and Max Power stickers) with your mates, wearing baseball hats, listening to an incredibly loud stereo, in an attempt to impress and then impregnate the local 14 year olds.
'Jon's gone mewling in his Nova. I saw him doing doughnuts in the Iceland carpark. The local chavettes seemed very impressed.'