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.'
Welsh valleys slang, believed to either have originated from, or given rise to, the 'can of lager/alcopop for £1 until 7pm' promotion that Merthyr's former premier nitespot The Zone used to run. Now means much the same as sound and safe in the complimentary 'cool' sense, expressing one's admiration for another individual/thing.