noun

• (mythology) A creature with the head of a person and the body of an animal, commonly a lion.
  "[W]hatsoe'er of strange / Sculptured on alabaster obelisk, / Or jasper tomb, or mutilated sphynx, / Dark Æthiopia in her desert hills. / Conceals."
• (figurative) A person who keeps their thoughts and intentions secret; an enigmatic or impassive person.
  "...it is clear that judges are no longer required to be as passive as they once were; to be what I call sphinx judges. We now not only accept that a judge may intervene in the adversarial debate, but also believe that it is sometimes essential for him to do so for justice in fact to be done."
• (dated) A mandrill (Mandrillus sphinx), formerly classified as a baboon, and called sphinx baboon.
• (euphemistic, rare) A sphincter.
  "Constance said boastfully, ‘My sphinxes are strong and in good repair. I order you to come.’"

verb

• (transitive) To decorate with sphinxes.
  "a marble sphinxed chimney-piece"
• (transitive, intransitive) To adopt the posture of the Sphinx.
  "A hot lion with a very bloated stomach … will adopt either a sphinxed or a squatting posture which takes some of the weight off its belly."
• (transitive, intransitive) To be inscrutable, often through silence.
  "The sphinxèd riddle of the Universe, Nature's unsolved enigma, who may prove?"
• (intransitive) To behave in a manner characteristic of the Sphinx.
  "Perhaps Nature is sphinxing us on purpose. Whatever her objects may be, perhaps she gets her work done better when she appeals to our gambling instincts. If you knew for certain exactly how your marriage was going to turn out[…]"

name

• (Greek mythology) One of the many offspring of Typhon and Echidna, a winged lion-like creature with a woman's face, who committed suicide out of frustration after Oedipus managed to solve her riddles.