Aha! That explains something for me - in my childhood I had a passion for the Middle Ages and read everything I could find about anything remotely medieval, including falconry. When a medieval falcon broke a main wing feather, the falconer would mend it by soaking an iron needle in brine or vinegar to make it rust, and using it as a splint for the feather till it healed. ( I gather falconers still do this, but these days they use carbon fibre and superglue.) This procedure was and is known as imping - I naturally assumed that this was an unrelated word to imp meaning a little devil, but clearly it isn’t.