[CRConrad]

Yup, definitely easiest. (That's how I first got it too.) Dunno if it's ethymologically correct, though; that depends on what the roots of "pain" are.

Since it's explained further down the page that it comes from an earlier loan from Latin "peona", "punishment", I suppose there is kind of a cognate in English itself from the later loan of the same word that has lead to "penalizing".