***This talk contains examples of obscene, offensive and controversial English vocabulary.*** ***Warning: This talk contains images of Aboriginal and Torres Straight Islander persons, who may be deceased.*** Everyone is familiar with some words that are generally avoided in polite conversation. Modern readers of the Harry Potter novels know who is referred to as ‘He Who Must Not Be Named’ just as readers of the Hebrew Bible understand the representation of the name of God through four letters which cannot be spoken. What can and can’t be said varies across time and place: to the Victorians ‘trousers’ were ‘unmentionables’. In this talk for the Cambridge Festival of Ideas James Clackson, Professor of Comparative Philology, shows how processes of linguistic taboo work across different societies, and how they change over time. He demonstrates how some of our current English vocabulary reflects our ancestors’ avoidances of unspeakable words. This talk contains examples of obscene, offensive and controversial English vocabulary.