Thingamajig's origination is unknown but maybe the most well known place would be sim city 2000 where one of the industrial buildings where named "Thingamajig". Thingamajig describes something you have no idea how to describe.
Person1: Oh you know, uh that thingymajiggeramabobaritoish thing.
Person2: No I don't. Perhaps you should stop being so retarded and speak fucking english.
an adverb that is used, rather humorously, to denote the manner in which a thing is done that cannot be understandably described; more or less equivalent to 'somehow or the other'.