Indescribable or unutterable, an unspeakable taboo.
Dated back to the late 1400’s, this adjective originates in the Old French ineffable or directly from the Latin ineffabilis which is a combination of the prefix ‘in‘ meaning ‘not’ and effabilis meaning speakable.
Why this word?
Having another word for unspeakable is always great, but this one is furthermore interesting for two reasons. First of all, the word ‘fame’ also stems from this root as the Latin word fama means talk, rumor, report or reputation. Thus that one who is constantly talked of, is a celebrity, holds fame and is also famous.
Secondly, this same word but in its plural form, ineffables, was for a short time around the early 1800, used as euphemism for trousers (The Routledge Dictionary of Historical Slang by Eric Partridge). Now you tell me, why is it that a euphemism for ‘trousers’ was even needed?!
How to use ineffable?
In the exact same what you would use unspeakable or a taboo. For example, human discharges are ineffable when seated at the dinner table. Yes, bad one, I know… An easier one would also work- when living in an orthodox house, many things are ineffable.