Definition of "Yogi"

I garaged the car and went to Aunt Dahlia's sanctum to ascertain whether she had cooled off at all since I had left her, for I was still anxious about that blood pressure of hers. One doesn't want aunts going up in a sheet of flame all over the place. She wasn't there, having, I learned later, withdrawn to her room to bathe her temples with eau de Cologne and do Yogi deep-breathing, […]

1960, P[elham] G[renville] Wodehouse, chapter XIX, in Jeeves in the Offing, London: Herbert Jenkins