Aramaic "rock" is kepha, which is a much more likely origin for Cephas, since that was the vernacular tongue of Palestinian Jews. It's probably also a pun on cephal, Greek for "head".
But which would have been derived from which: the Aramaic "kepha" from the Hebrew "keph", or vice versa???
And since the vernacular of the Jews around Jerusalem at that time was Hebrew and not Aramaic [as evidenced by the absence of Aramaic on the inscription on the cross], and since the language of the Law, Prophets, and Writings was Hebrew and not Aramaic [except for a few chapters of Daniel], and since Jesus was the Jewish Messiah, not the Syrian or Chaldean one, then Peter's nickname more than likely means "a hollow rock".
If Jesus had meant a huge or substantive rock for Simon, then the Hebrew words "Selah" or "Tyrus" would have been more appropriate.