'diglossic' originates from Greek, specifically the word 'diglossia' (Greek: διγλωσσία), where the prefix 'di-' meant 'two' and 'glōssa' meant 'tongue' or 'language'.

'diglossic' developed from the noun 'diglossia' (borrowed into English from Modern Latin/Greek in linguistic usage). The Greek διγλωσσία ('diglossia')—literally 'two tongues'—entered linguistic literature in the 20th century and the adjective 'diglossic' was formed in English to describe that condition.

Initially it referred literally to the idea of 'two tongues' (two language varieties); over time it has become the technical adjective describing the sociolinguistic condition (i.e., 'relating to diglossia').