Fester
late Middle English: from the rare word fester ‘fistula’, later ‘festering sore’, or Old French festrir (verb), both from Old French festre (noun), from Latin fistula ‘pipe, reed, fistula’.
wiktionary
From Old French festre (cognate with Italian fistola, Occitan fistola, Spanish fístula), from Latin fistula. The verb is derived from the noun, while the “condition of something that festers” noun sense is derived from the verb. Doublet of fistula.
etymonline
fester (v.)
late 14c., of wounds, "to become ulcerous, suppurate," from festre (n.) "a fistula" (c. 1300), or from Old French verb festrir "ulcerate, fester," from festre (n.) "small sore discharging pus." The nouns in Old French and Middle English both are from Latin fistula "pipe, ulcer" (see fistula). Related: Festered; festering; festerment.
