From a Russian pet form Соня (Sónja) of Sophia, from the Ancient Greek word for "wisdom". It was introduced into English through 19th century Russian literature and taken into general use in early twentieth century.
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The only young people remaining in the drawing room, not counting the young lady visitor and the countess' eldest daughter (who was four years older than her sister and behaved already like a grown-up person), were Nicholas and Sonya, the niece.
As he spoke he kept glancing with the flirtatiousness of a handsome youth at Sonya and the young lady visitor.
Hardly had Boris gone than Sonya, flushed, in tears, and muttering angrily, came in at the other door.
Sonya, muttering to herself, kept looking round toward the drawing-room door.