That dictionaries engage with truth is a commonplace in the history of lexicography. ‘Truth’ shows how difficult that quality can be to achieve. Factual errors can be revealed in relatively objective ways. Other entries, however, are more complex, revealing how definitions (and the truths they are assumed to convey) can be aligned along the fault-lines of belief and behaviour, of moral or political authority, or of bias and prejudice. Dictionaries are bound to time and place in ways that repeatedly remind us of the surprising mutability of ‘truth’, and of the radical changes that can intervene in what might be deemed a ‘good’ (or indeed a ‘bad’) definition.