Unspeakable means too dreadful to be put into words. Unspeakable cruelty, unspeakable suffering, an unspeakable crime.
It is one of the strongest words available and it is weakened by casual use, so it belongs to atrocity rather than to inconvenience. Literally the -able means capable of being spoken, so unspeakable means what cannot be said rather than what should not.
1adjectiveunspeakableToo dreadful to be put into words.
“The prisoners suffered unspeakable cruelty in the camps.”
unspeakable is a quality.
Un- plus speakable, from Old English sprecan, to speak. The compound is recorded from Middle English.
Middle English
unspekable
English, today
unspeakable
From Middle English unspekable, equivalent to un- + speakable.
Etymology from Wiktionary, CC BY-SA