the.com/mother tongue

means The first language you learned as a child, the one your mind runs on before you even think about it.

from A straightforward compound built on an old metaphor: language as something nursed, passed from mother to child along with milk and lullabies. Versions of the phrase show up across many European languages (Latin's lingua materna among them), suggesting the mother-as-source-of-speech image is ancient and widespread rather than owned by any one tongue. English picked it up in the medieval period, and it's stuck ever since — a rare case where the metaphor is almost too literal to question.