‘Semilingualism’ is one of the most questionable theories produced in the language sciences. Yet, little is known about its origins. We present a critical account of the history of semilingualism, tracing its roots in the work of Nils Erik Hansegård, (1918–2002), inaugural chair of Sámi at Umeå University (1975–1979), who developed a theory of semilingualism (halvspråkighet) in the 1960s. We show how Hansegård theorized semilingualism using ideas from Nazi German linguistics, producing an unforgiving theory of linguistic pathology directed at minoritized bilinguals in Sweden's far north.