The purpose of this paper is to offer a critical review of the sociolinguistic and ideological assumptions upon which the Castilianist theory of the origin of the Galician gheada is based. To this end, we will review the concepts of diglossic bilingualism and prestige, fundamental to this theory, to point out their limited applicability to the historical period to which they are applied (the 18th and 19th centuries), given the social structure and cultural situation of 18th and
19th-century Galicia. Likewise, we will endeavour to demonstrate how this theory derives from a concept of linguistic interference characteristic of xenophobic purism, resorting to naturalisation tactics to present its doctrine as common-sense truths shared by everyone, and contributing to spreading the stigma of this social variety by identifying the causes of the language change with lack of education, ignorance, or incapacity of the lowest social classes to pronounce a particular sound, thus providing a theoretical basis to the existence of practices of discrimination and social exclusion based on language use.