Kadigan: A placeholder word such as whatchamacallit, thingamajig, yada-yada, oh-dark-thirty, or foo.
Kadigan (sometimes spelled cadigan) seems to have originated in the 20th century, possibly with the American writer, poet, and editor Willard R. Espy, who wrote An Almanac of Words at Play. The term is included in Dictionary of American Slang (first published in 1960 and now in its third edition), whose original editors connected it to gin, as in cotton gin.
Kadigans are used in virtually every language. For some fascinating word lists, see this Wikipedia article, which also includes this helpful clarification:
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.
(Hat tip: Kottke.)