Having been challenged by my significant other, after using the word "doohickey" in common parlance, with a $10 bet that the word doesn't exist in the dictionary, I do some digging....

doohickey: see "doodad" (Websters , online)

doodad:  (paraphrased) any, usually small, article which the speaker cannot remember the proper name for (Websters , online)

ok, so I win the bet. But let's dig some more and check out what wikipedia.com has to say about "doohickey"

some excerpts from Wikipedia's placeholder page (?!) for "doohickey", redirected from "Doohickey" (?!)

The term is described, yup, you guessed it, as a substitute for some other word the speaker can't remember. Then the page goes on, rather lengthily even for my tastes, to catalog words in many different languages that mean essentially the same thing: whatever the speaker wants it to mean.

My favorites?

truc, French for "trick", used like doohickey (along with quelque chose, meaning "something", which when in college all pretendeurs to thrones would pepper vigorously into their vocabulary to make it seem like they spoke both haute-slang and book French)

Uncle Tom Cobley et al, used as a substitute for, assumedly, an actual lengthy list of people (assumedly, also, by me, not qualified to be among a list of personnes)

Some more fun... here's an excerpt from the page itself, describing in detail some of the exquisitely essential differences between certain of these substitute words:

Thingamajigs are typically specialized devices which have a limited number of uses or a single specific use. The term is typically employed by one whose experience with the use of the object is nonexistent or very limited. Regular users of such devices would never refer to them as thingamajigs or any of the related terms listed below.

A thingamajig is different from a widget, in that a widget is an actual, but not yet named or constructed, mechanical component. It is also different from a gadget, in that “gadget” is the generic term for a superfluously useful device, such as a remote garage door opener, whose name is easily remembered.

Even among the world of otherwise nameless things referred to by placeholder names, there is a hierarchy of specificity. "Thing", as its name implies, is universally applicable. It is likely, however, that a "gizmo" involves some minor degree of technological sophistication, connoting as it does some mechanical or electronic aspect.

Most of these words exist in the less formal register of the English language. In more formal speech and writing, words like accessory, paraphernalia, artifact, instrument, or utensil are called into play; these words also refer to things made by human hands without getting specific about their form or function. These words also differ slightly in usage: artifacts are usually found objects of indeterminate age and purpose, while utensil suggests cutlery.

These words have been in regular use since at least the nineteenth century. Edgar Allan Poe wrote a short story entitled The Literary Life of Thingum Bob, Esq., showing that particular form to be in familiar use in the United States in the 1840s. In Gilbert and Sullivan’s The Mikado, W. S. Gilbert makes the Lord High Executioner sing of a "little list" which includes:

. . . apologetic statesmen of a compromising kind,
Such as--What d’ye call him--Thing’em-bob, and likewise--Never-mind,
and ’St--’st--’st-- and What’s-his-name, and also You-know-who--
The task of filling up the blanks I’d rather leave to you.

 Similarly, Dictionary.com references "doodad", "gizmo" and "gadget", adding "dingus" and "thingumbob" to their collection while defining Doohickey.

Interestingly, place names also apply to the substitutable word category. When in college, my pals and I tended to use BFE (which is listed on the Wikipedia page) to describe wherever our cars were and where we were not. When I was growing up, my mother used to refer to far away places that we weren't going to go to today as "East Overshoe" and similar other made up names. These were not on the Wikipedia list. My new fave from the inexhaustible list is:

Up the Boohai (approximately "boo-eye") in New Zealand, occasionally given as, Up the Boohai hunting pukekos with a long handled shovel. The Boohai is a fictitious river. It is used to indicate that the answerer does not wish to respond to any question involving "where?". Up the Boohai can also indicate that plans are apparently ruined or an item is extremely non-functional. 

I'm still waiting to collect on that bet....