‘Deadnaming’ is the act of referring to trans people by the names assigned to them in infancy in cases where they have rejected those names. In this paper, Judith Butler’s (1997) theorising of injurious speech is united with Mary Bucholtz and Kira Hall’s (2005) tactics of intersubjective identity formation to conceptualise deadnaming as a speech act with a harmful perlocutionary force, which may be employed to denaturalise or illegitimate a trans person’s acts of self-determination. This is demonstrated by an analysis of entries on the crowdsourced website Urban Dictionary that were posted in response to American celebrity Caitlyn Jenner’s public coming out as a trans woman in 2015. Attempts by Urban Dictionary’s users to ‘redefine’ Jenner’s names both reveal the cisnormative assumptions that motivate deliberate acts of deadnaming and expose anxieties about how the public visibility of trans people threatens the alleged naturalness of those assumptions.