so now we have a sesquipedalian word for these characters.

Liar, Liar, Pants on Fire

A guy who boasts he's a liar
Is an abydocomist in dire
Trouble you see
'Cause naturally
No-one'll believe him when his pants are on fire.

I think[?] this is the moral of the nursery rhyme, tsuwm, but I can't find it online.