Other forms: doohickeys
A doohickey is a "whatchamacallit" or a "doodad." In other words, it's something whose exact name you don't know or can't remember, like the doohickey you use to pry open that drawer that always sticks shut.
You can add doohickey to your list of all-purpose placeholder words, along with "thingamabob" and "gizmo." These terms are useful when the actual name of the item escapes you: "Hand me that doohickey on the table. Not that one, the little doohickey that screws in this bolt!" Doohickey was first used as U.S. Navy slang in the early 20th century and defined in 1925 as "an airman’s term for small, detachable fittings."