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]
来自拉丁语fistula, 导管，溃疡。来自PIE*bheid, 劈开，裂开，词源同bite, boat.比喻用法。