Peer tutoring is viewed as a valuable component of additional language learning due to the presence of a more knowledgeable interlocutor. Yet researchers and language program directors alike often ignore the linguistic and cultural differences that peer tutors possess, instead categorizing them homogeneously as ‘experts’ or ‘native speakers.’ In this article, I use a case study to closely examine how knowledge is negotiated in one peer tutoring cohort. Grounding my analysis in ethnomethodological and linguistic anthropological notions of epistemics and expertise, I show how one peer tutor drew from various embodied, artifactual, and historical resources in order to negotiate lexical gaps and position herself as an expert in the target language. At the same time, I demonstrate how essentialist ideologies help construct a language expert by highlighting the learners’ alignments to the tutor's epistemic stance as knower, even when faced with conflicting information. These findings question the ways knowledge and expertise are traditionally perceived in peer tutoring and other additional language learning contexts, emphasize the need for training peer tutors in cooperative learning methods and articulating their knowledge with that from the classroom setting, and highlight the complex ideologies that surround the ‘right to know’ a target language.