I can’t remember where I came across this one now, but it’s an interesting pair that I thought got confused in certain circumstances regarding the stirring up of unrest. But wouldn’t you know it – a different picture emerges when I look into it more deeply, and this joins the select ranks of words such as decimated or gunnel that I am not really allowed to be as picky about as I’d like to be!).
To ferment as a verb means to undergo the process of fermentation (“His beer was fermenting nicely and would be ready to drink in a few weeks”) or to stir up disorder. A ferment (as a noun) means social unrest and agitation.
To foment, as a verb, means to instigate or stir up something like strife or revolution – i.e. the same as the secondary meaning of ferment as a verb. What a shame! I think we should still use this for revolutions and ferment for beer, myself (to foment has a nice archaic meaning of bathing a part of the body with a warm or medicated lotion, or fomentation. I doubt any of us will ever use that, but it’s quite jolly, in my opinion).