Background: Long-standing calls to infuse technical language in teaching--what we call the Professional Language Project--have been revived in recent years along with the core practices movement in teacher education. The Professional Language Project has been identified as a desired outcome of research and a potential benefit to teacher education. Objective: Drawing on sociolinguistic studies of teachers' sensemaking, we critique the Professional Language Project to show its limits in making the intended contribution to teaching and teacher education. Research Design: This analytic essay uses a practice perspective on both language and teaching to interrogate the premises of the Professional Language Project. Specifically, we hold up its goals against empirical findings about how teachers use language to make sense of instructional decisions in their workplaces. Conclusions: Empirical studies of teachers' in situ language use point to two fallacies in the Professional Language Project. First, the presence or absence of technical terms in teachers' talk does not relate to the depth of their sensemaking or instructional sophistication, indicating that technical terms do not accomplish the conceptual goals that some Professional Language Project advocates suggest. Second, a prevailing common-sense discourse culture in teaching often results in conceptual slippage in the use of technical terms, leading words to be absorbed into existing conceptual systems more than they catalyze new understandings.