Old English fenol, finul, finol "fennel," perhaps via (or influenced by) Old French fenoil (13c.) or directly from Vulgar Latin *fenuculum, from Latin feniculum/faeniculum, diminutive of fenum/faenum "hay," probably literally "produce" (see fecund). Apparently so called from the hay-like appearance of its feathery green leaves and its sweet odor.
1870 in the anatomical sense, noun of action from Latin fenestrare, from fenestra "window, opening for light," a word perhaps from Etruscan (see defenestration). Meaning "arrangement of windows" as a design element in architecture is from 1846. Related: Fenestrated.