I might be wrong but I thought it stemmed from the US of A where gangs/chavs would hold guns out of direct sight (like a bank robber) as a threat?
Either way, it's disgusting and looks ridiculous.
Probably the same gangsters who sit down the shopping parade on their sisters hair dryers wearing full face crash helmets "lookin 'ard"
It's a self comfort mechanism, kinda pathetic.
I was thinking the same, it's like the chav version of sucking your thumb.
You see as a child and a teenage relaxing at home it was normal *for me anyway* to cup whilst watching TV etc.
Knuckles hang too low, keeps them elevated.