How can something be ‘unspeakable’?

This Thursday’s Thought from Word of the Day comes as promised from the Vatican itself (via an intermediary): how can something be ‘unspeakable’ if we can speak of it?

Yes, we managed to secure the good offices of the Holy See – even though the Prime Minister, Leader of the Opposition and James Murdoch all clamoured for today’s word to be ‘hindsight’. So last year, chaps: ‘with hindsight’ was Friday’s Phrase in February 2010. I expect some of them wish they could wind the clock back to then – which is all the more reason for them to subscribe to Word of the Day. (I don’t see any of them on the Twitter followers list….)