Single word of "Senseless words"

The possible candidates I have identified (partly with the help of OneLook.com) are conspiracies, fallacies/pathetic fallacies, fancies, fantasies, idiosyncrasies, illiteracies, inconsistencies, lunacies and redundancies; and some of those are pretty marginal possibilities at that.

If none of them ring a bell, then I think your memory is at fault.

I would think idiocies would go here. It means

foolishness or senselessness; stupidity; a foolish act or remark (Collins)

e.g.

He was complaining again about the idiocies of the people he works for. (M-W)

The sentence you are looking for may be

You are speaking idiocies.

It is not a frequent expression, but it is colloquial. That's why it is rarely found in written texts.

e.g.

Trondr, I called you here to offer me good suggestions, not to speak idiocies. (reddit)

To speak idiocies to children but not to tell them truths, to say polite nothings to women but not truth... (quote attributed to Nietzche - GBooks)

I can't think of an expression that would include the preposition in here ("speaking in idiocies" doesn't seem right).