father tongue

Meanings

1. A separate language for expressing ideas, as opposed to the vernacular (mother tongue) which is employed for everyday speech.
2. The form of language acquired through education and reading, as opposed to the dialect one grows up speaking; educated or formal language.
3. A second language that one speaks fluently.
4. The language spoken by one's father, when it differs from that spoken by one's mother.

Etymology

From father + tongue (“language”), modelled after mother tongue.