Green’s Dictionary of Slang

(Aus.) anything or anyone that one has forgotten the name of.

1981		R.A. Spears Sl. and Euphemism 163: A thing or tool [...] whosermybob. [Ibid.] 423: whosermyjig a gadget; a name used when the correct name is forgotten or unknown.
1999		G. Seal Lingo 72: There are even Lingo names for those things we cannot remember the names of- whatsit, dooverlackey (or just doover), whatchamacallit, thingumebob, thingy, whosamajig, whatasamajig, thingumajig, whatsit, what’s-his-face and whosemawhat.