silly billy 
noun informal, chiefly British
A silly or foolish person: don’t be a silly billy...
- This is a very good alternative crop for farmers and I do not want it to be jeopardised by some silly billies in society.
- The dressing stations will be busy enough without having to patch up silly billies who didn't wear sensible shoes.
- The ‘silly billy’ was rewarded with the Bunker Banana Booby for this slip.