This is a word that is common enough, but it offers no clues as to its origins – it doesn’t look or sound like any particular language or time. My Chambers dictionary of many years ago defines it as ‘any small ingenious device; a what-d’ye-call-it, origin obscure.’
the current Oxford Dictionary offers little more, but it does claim it has a nautical origin, the first citation being:
R. Brown Spunyarn & Spindrift xxxi. 378 Then the names of all the other things on board a ship! I don’t know half of them yet; even the sailors forget at times, and if the exact name of anything they want happens to slip from their memory, they call it a chicken~fixing, or a gadjet, or a gill-guy, or a timmey-noggy, or a wim-wom—just pro tem., you know.
But rummaging through some old documents, I found the following, accompanying a photograph of a sailing ship, moored at the Mardyke pub, on the Floating Harbour popular with bikers:
“This is a “Gadget”, the origin of the word meaning any useful accessory. It was the invention of a Bristol stevedore, being a barge with a donkey-engine supplying power for lifting tackle on a sailing ship. This full-rigged ship has been recognised as from North America her name “Minnie..” She is tied up at the Mardyke, where training ships “Daedalus and “Flying Fox” have been in consecutive service singe 1861.
Bristol history is littered with outrageous, unfounded and quite often plain nonsensical claims, but this is from the OED again, wiht the first local use of the word in terms of unloading vessels, so for a change,they might be right, but so might many other nautical places. :