Some time back Dave T said in part: "Johnny Conkeroo, John the Conkeroo is an adaptation of John The Conqueror and refers to a small statue used as a good luck charm. There are a lot of references to charms, magic, voodoo etc. in blues."
While this explanation isn't wrong, down here (in the South) the phrase is usually "High John the Conqueror" and refers to a branched ginseng root that has the abstract appearance of a human figure. When dried it was left intact to be used as an amulet or charm or was pulverized into a powder to be added to food or beverages, supposedly to give the consumer a leg up on good health. There used to be a concoction advertised as a daily tonic on radio and local print media back in the 50s that listed among its ingredients "High John the Conqueror root" and in the voice ads on radio this ingredient was almost always mentioned, apparently to give the tonic some credibility as a medicinal preparation. I haven't heard this tonic mentioned in many decades so I don't know if it's still being made but I suppose it would be relegated to the "health foods" or "herbal supplements categories now and couldn't be sold as a true OTC medicine.