It's actually only assumptions about the word, but based on the fact that it would extremely weird for people to borrow someone else's word for something as familiar as a pig. Without wanting to take this thread too far in another direction, Celtic lost the proto-Indo-European P, so to take two modern examples Welsh llawr is cognate with English floor and Irish lán is cognate with French plein. That doesn't mean there are no Ps in Celtic, only that an original PIE P wouldn't be there. PORCOM obviously has one. All languages lost PIE k' but the Centum (Italic, Celtic, Germanic) turned it into a K sound while the Satem (Slavonic, Persian, Indic) turned it into an S sound. So because PIE *pork'o has become porcom not *porsom we can tell this.
EDIT - that's "Centum" not "Centrum" - they didn't name a branch of Indo-European after vitamin supplements...
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