Emily Dickinson Lexicon

FA-NAT'IC, or FA-NAT'IC-AL, a. [L. fanaticus, phanaticus, from Gr. φαινομαι, to appear; literally, seeing visions.]

Wild and extravagant in opinions, particularly in religious opinions, excessively enthusiastic; possessed by a kind of frenzy. Hence we say, fanatic zeal; fanatic notions or opinions.