Definition of "thingumbob"

Don’t you know what’s-his-name’s, close to the thingumbob?

1878, Henry James, An International Episode‎

[…] his precious helpmate sat grinning at the mischief she had occasioned, like an idiot, asking Colonel Lamborn to help her to some of them there thingumbobs out of the silver what-d’ye-call’um. These thingumbobs happened to be no other than a service of West India sweetmeats in a superb fillagree epergne, but as Samuelina understood not the name of any foreign fruit, or the vessel appropriated to contain it, it was not to be wondered she adopted the above titles, which almost convulsed Mrs. Lamborn with laughter, had not her good sense and politeness restricted any sarcasm an arrogant sprig of quality would have taken advantage of.

1810, [anonymous] […], chapter XXVIII, in Splendid Follies. A Novel, […]. Founded on Facts., volume III, London: […] J[ames] F[letcher] Hughes, […], pages 129–130