My goodness! Do I have a twisted mind!
I have not read the novel. However, "thinga-ma-jig" is an old familiar word in my life. It is quite commonly used in daily conversation when one is speaking of a particular item, and cannot immediately recall it's proper name. For example: "Will you please hand me that...thingamajig?" One may be pointing direcly toward it, and the other party obviously knows what it is.
I forgot a most important use of "thingamajig" - - when looking under the hood of a vehicle.