Metasyntactic variable

This is a thing, you know, doodah. Whatsisface, had it on the tip of my tongue a moment ago, thingummy, doobrie, yeah right, it's a Wossname!

These constituted virtually the entire vocabulary available to Eric's... bird-shaped thing, ...big beak, Parrot.

In Men at Arms, Gaspode, another animal blessed with human speech and limited vocabulary items, also employed, and in fact specifically referred to, the metasyntactic variable, in order to get a point across.