This card is a reference to the ancient Greek tale of Oedipus. When travelling across cities, a sphinx blocked traveler's paths and demanded he answer a riddle (It was: "What creature walks on four legs in the morning, two in the afternoon and three in the evening?"). If the person in question gave a wrong answer, he would be devoured. If the person in question gave the correct answer, which was a human, the sphinx would let him pass. Similarly, if the opponent guesses incorrectly, their attack fails.
An alternate interpretation of this card is the legend of Thutmose IV. As a prince, he fell asleep under the head of the Sphinx, whose body was buried in the sand. He dreamt the Sphinx instructing him to excavate it and in return, the Sphinx will make him Pharaoh, in spite of not being next in line. Upon waking, he did as he was told and eventually, the Sphinx was dug up. Thutmose then became Pharaoh and he erected a stela with an inscription of his dream between the Sphinx's paws.