fern (n.) Look up fern at Dictionary.com
Old English fearn "fern," from Proto-Germanic *farno- (cognates: Old Saxon farn, Middle Dutch vaern, Dutch varen, Old High German farn, German Farn).

Possibly the word has a prehistoric sense of "having feathery fronds" and is from PIE *por-no-, which has yielded words for "feather, wing" (cognates: Sanskrit parnam "feather;" Lithuanian papartis "fern;" Russian paporot'; Greek pteris "fern," pteron "feather"), from the root *per- (2) "to lead, pass over" (see petition (n.)). The plant's ability to appear as if from nothing accounts for the ancient belief that fern seeds conferred invisibility (1590s).