What does Boxing Day have to do with boxing?
before vowels petr-, word-forming element used from 19c., from comb. form of Greek petros "stone," petra "rock" (see petrous).
word-forming element used from mid-20c. to mean "of or having to do with petroleum products," from petroleum.
petro- or petri- or petr-pref. Rock; stone; stone-like hardness: petrous.