Navvy Gravvy

Unlike Itsy Bitsy and Teeny Weeny, Navvy Gravvy doesn't refer to just a small size but to a small amount. A navvy gravy is a very small amount, a little tiny bit, a very fine shaving. In common use within the Royal Dockyards where very small adjustments have to be made, by shipwrights and shipfitters, to customise components in order to get them to fit correctly. Thought to date from around the mid-nineteenth century when many tasks in shipbuilding were still carried out by 'eye', although the origins or the words themselves are not entirely clear, it is still in use today where non-standard sized items have to be adjusted to fit.
As an illustration; if a hole is drilled in a piece of metal to take a bolt, the hole is the correct size but the bolt doesn't fit. The drill is then run through the hole again and the bolt fits. Careful examination may reveal a few grains of metal dust or the tiniest finest curl of swarf removed by the drill, that tiny amount is a navvy gravvy.