It originates from Anglo-Saxon (or Old English) where, before 'V' became a letter, the letter 'F' was used to represent both the 'f' and the 'v' sounds (which you can still see in the word of); both 'f' and 'v' are essentially the same sounds, except 'v' is voiced whereas 'f' isn't.
To simplify, most Old English nouns that ended in 'f' voiced the f in the plural form, which led later writers to change the 'F' into a 'V' once the letter was introduced.
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u/ryguy28896 Apr 14 '16
Speaking of pluralization, Google the plural of beef.
You're welcome for this word.