"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]
late 14c. (mid-14c. in Anglo-French), "one of the common people, a member of the third estate," agent noun from common (v.) "participate in common, associate or have dealings with" (mid-14c.), from common (adj.). From mid-15c. as "member of the House of Commons."
"one who or that which communicates," 1660s, from Late Latin communicator, agent noun from communicare "to share, divide out; communicate, impart, inform; join, unite, participate in," literally "to make common," related to communis "common, public, general" (see common (adj.)). Related: Communicatory.
"member of the lowest class or the common people," 1530s, from Latin plebius "person not of noble rank," from adjective meaning "of the common people" (see plebeian (adj.)).