This paper examines the relationship between English-medium instruction and the coloniality of knowledge in Nigeria. While debates surrounding language of instruction have traditionally focused on educational outcomes, linguistic competence, and national cohesion, less attention has been paid to the epistemological consequences of privileging English as the dominant language of schooling. Drawing on language policy studies, critical applied linguistics, linguistic imperialism, and decolonial theory, the paper introduces the concept of colonilinguistics to explain how colonial language hierarchies continue to shape knowledge production and validation in postcolonial educational contexts. Using Nigeria as a case study, the article argues that English operates not only as a medium of instruction but also as a medium of epistemic power, influencing whose knowledge is recognised, whose voices are heard, and which intellectual traditions are considered legitimate. The paper concludes by proposing a multilingual and epistemically inclusive approach to education that challenges the coloniality embedded within contemporary language policy.