According to "Lost Splendor", written by Prince Felix Youssoupoff, Felix Elston (his paternal grandfather) "... was said to be the son of Frederick William IV, King of Prussia, and Countess Tiesenhausen, maid of honor to the Empress Alexandra, the King's sister. "
More: "...their secret love affair resulted in the birth of a son, Felix Elston. Spiteful contemporaries construed this name as the contraction of three French words: elle s'e'tonne (she is astonished) which supposedly expressed the feelings of the young mother."
Hope this helps,
Tasha