To form pus or decay; to become worse over time; or to grow increasingly bitter or resentful.
From Latin 'fistula' (ulcer, pipe-like wound), through Old French 'festre.' The word originally described literal wounds that infected, then became metaphorical for emotional wounds.
'Fester' comes from 'fistula,' which is actually a medical term still used today for abnormal pipe-like channels that form in the body from infection—so when we say a problem 'festers,' we're describing it like an infected wound growing worse.
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