Sentence:  Ramanujan has also argued that many Hindu men have both a mother tongue (the everyday language, such as Tamil, spoken by women downstairs, in the back,  in the kitchen) and a <WSD>father tongue</WSD> (once Sanskrit, more recently English, the literary lingua franca spoken—or at least discussed—by men in the front rooms). 
Meaning of the word marked with <WSD> tags:  A separate language for expressing ideas, as opposed to the vernacular (mother tongue) which is employed for everyday common speech.