'mother-tongue' originates from Old English, specifically the words 'mōdor' and 'tunge', where 'mōdor' meant 'mother' and 'tunge' meant 'tongue' (speech, language).

'mother-tongue' appeared in Middle English in forms such as 'modertunge' and developed into the modern compound 'mother-tongue' through regular phonological and orthographic changes.

Initially it literally referred to the 'mother's tongue' (the speech of the mother); over time it came to mean a person's native language or first language more generally.