Is it ensure, insure, or assure?
early 15c., "monster of Greek mythology," from Latin Sphinx, from Greek Sphinx, literally" the strangler," a back-formation from sphingein "to squeeze, bind" (see sphincter).
Monster, having a lion's (winged) body and a woman's head, that waylaid travelers around Thebes and devoured those who could not answer its questions; Oedipus solved the riddle and the Sphinx killed herself. The proper plural would be sphinges. Transferred sense of "person or thing of mysterious nature" is from c.1600. In the Egyptian sense (usually male and wingless) it is attested from 1570s; specific reference to the colossal stone one near the pyramids as Giza is attested from 1610s.
In the story of Oedipus, a winged monster with the head of a woman and the body of a lion. It waylaid travelers on the roads near the city of Thebes and would kill any of them who could not answer this riddle: “What creatures walk on four legs in the morning, on two legs at noon, and on three legs in the evening?” Oedipus finally gave the correct answer: human beings, who go on all fours as infants, walk upright in maturity, and in old age rely on the “third leg” of a cane.
Note: The sphinx of Greek mythology resembles the sphinx of Egyptian mythology but is distinct from it (the Egyptian sphinx had a man's head). (See under “Fine Arts.”)