Words like “thingamajig,” “whatsit” and “doohickey” are called “placeholder names” or kadigans.
The origin of “kadigan” isn’t clear. Some suggest it could somehow be related to the generic word “gin,” as in a “cotton gin,” (not related to the drink) which is a shortening of the word “engine” and took the place of engine in relation to the machine. But it’s most likely related to the Irish or Welsh name Cadigan, perhaps after someone who was once well-known for using them frequently—which could arguably make the word kadigan a kadigan since it stands in for the term “placeholder name.”
That’s because placeholder names or kadigans don’t just have to be nonsense words like thingamabob. The generic names John Doe and Jane Doe are also examples of kadigans because they take the place of actual names. Apple uses Johnny and Jane Appleseed as kadigans in its products.
You could also consider a recurring fictional brand to be a kadigan: For example, the company Acme in classic Warner Brothers cartoons. Acme means “the highest point” in Greek and is a plausible company name—in fact, it was inspired by the names of several real companies—albeit an ironic one given the constant failure of their products.
Another example is the fictional cigarette brand called Morley, which is based on Marlboro, and shows up in shows including The Twilight Zone, Friends and Seinfeld.
The generic latin lorem ipsum text used in publishing and graphic design is also an example of a kadigan. It is originally a corruption of a first-century BC text by the Roman statesman Cicero.
Kadigans can also be numbers: “Umpteen” stands in for any annoyingly large number, as in the phrase “for the umpteenth time.”
There are also geographical kadigans:
People use the word “podunk” to mean a town in the middle of nowhere. It comes from the name of the Native American Podunk people, who lived in a marshy, remote area of Connecticut that is now near Hartford County. In their indigenous language, podunk means “where you sink in mire.” The term podunk became a kadigan thanks to an 1846 humor column called “Letters from Podunk” in the Daily National Pilot, which was published in Buffalo NY. Writers like Mark Twain picked it up and further popularized it.
A similar example of a geographical kadigan is the “boondocks” or the “boonies” as a word for a rural area. Both come from the Tagalog word bundók, or “mountain.”
Kadigans can also be a bit racist—like when people add the suffix -stan onto a word to compare it to a place in the Middle East.
Useless Etymology 