Seems it wasn't Obelix's gravitas, Napoleon's archaeologists, or Mameluke artillery practice, but something else:

 Quote:
The nose was probably removed in the 8th century AD by a Sufi who considered the Sphinx a blasphemous idol, but all that can be said for certain, based on the tool marks that remain, is that it was deliberately pried off with chisels. The Sphinx's face, which in ancient times was painted dark red, was also decorated with a stone beard and displayed a sculpted cobra on its forehead, both of which have also fallen off. This may explain why for much of the Sphinx's later history, its face was interpreted as a woman's (link).

Ceci n'est pas un seing.