A “doohickey” is a small object or gadget, especially one whose name the speaker does not know or cannot recall. Synonyms are those well-used words: thingamabob, thingamajig, whatchamacallit.
It may seem as if a “doohickey” is a thing that’s too unimportant to have a name of its own, but no object is that unimportant. It’s just that there are so many doohickeys in the world that we can’t possibly know or remember the names for all of them. The word often refers to some small device, especially a mechanical one. For example, in The Tommyknockers, author Stephen King writes, “You’re almost done with this part. Just solder that red wire to that point to the left of the long doohickey.”
We don’t know where doohickey comes from, though the authorities usually point to its being a blend of doodad (a trivial or superfluous ornament) and hickey, which now is more usually a pimple or a love bite. However, at the beginning of the 1900s, “hickey” could be an odd person or something of little consequence.
The first recorded example appeared in the magazine Our Navy in November 1914: “We were compelled to christen articles beyond our ken with such names as ‘do-hickeys,’ ‘gadgets,’ and ‘gilguys’.”
And what is a “gilguy”? The Sailors’ Word-Book of 1867 says a gilguy is “A guy for tricing up, or bearing a boom or derrick. Often applied to inefficient guys.” The word “gilguy” is still in use in sailing circles for a gadget, or to refer to a line used as a temporary guy.
I’ve known a couple of inefficient guys, but the words used to describe them were usually profane rather than slangy.