'Majigger' is a shortened form of 'thingamajigger' (or 'thingamajig') - a word used for some object that we don't know the name of, but still need to mention and identify. Another word like this is 'whatis' (what is it). "I don't know what you call it, but if you turn this thingamajig (majigger) here, it should turn the volume up." "I see how this works, except for this majigger over here." "Oh, that's the latch for the battery door."