In the army, I was informed that 'jankers' or
'janglers' were the fetters used to constrain a
soldier or sailor awaiting punishment. The slang
term dates, I understand, from Napoleonic times.
Maybe so, Scylax, but the scholars at The Oxford English Dictionary have failed to come up with any such connection, I'm afraid.
They know that a 'janker' (singular) was a long pole on wheels used in Scotland to carry heavy loads and that 'jankers', in the sense required here, was first recorded in 1916.
It seems improbable, therefore, that they would have missed any sense of 'Napoleonic fetter'. Personally, I'd trust the Army for many things but etymology isn't one of them! Cheers
The problem is, Rass, that there is no evidence whatever that the word 'janglers' was ever used to mean 'fetters'. Even less - if that were possible - that it morphed into 'jankers' over time.
All folk etymology - or etymythology as some experts jokingly call it - sounds plausible. That's why vast numbers of people believe, for example, that the word 'posh' came from the opening letters of the phrase 'port out starboard home'. It didnt! But it certainly seems convincing.
If the folks at TOED, as I mentioned earlier, say the origin of a word is unknown, it's unknown.