Etymology
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fistula (n.)

"long, narrow ulcer," late 14c., from Latin fistula "a pipe; ulcer," which is of uncertain origin. Related: Fistular; fistulous (Latin fistulosus "full of holes; tubular").

No certain etymology. The best comparison seems to be with festuca "stalk, straw" and maybe ferula "giant fennel" (if from *fesula): the forms of a "pipe" and a "stalk" are similar. The vacillation between fest- and fist- occurs within festuca itself, and might be dialectal, or allophonic within Latin. [de Vaan]
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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.
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