thingumabob in British English
(ˈθɪŋəməˌbɒb) or thingummybob (ˈθɪŋəmɪˌbɒb)
noun informal a person or thing the name of which is unknown, temporarily forgotten, or deliberately overlooked
Also: thingumajig (ˈθɪŋəməˌdʒɪɡ), thingummyjig (ˈθɪŋəmɪˌdʒɪɡ), thingumbob (ˈθɪŋəmˌbɒb), thingummy (ˈθɪŋəmɪ) Word origin
C18: from
thing1, with humorous suffix