I stick by "um" as the word most often used by persons whose tongues have been caught in the headlights, but someone who knows French might mint a new bit of blabbery on the lines of faux pa (no matter how it's spelled). I thought one meaning "empty mouthed" or "clipped tongue" or something would be good. (I believe I've often heard my own tongue-tipped words referred to as "idiot" by loved ones.) As long as we're at it, what about one for spellings on the tip of one's fingers?

Binky