1520s, "insane person," from Latin fanaticus "mad, enthusiastic, inspired by a god," also "furious, mad," originally, "pertaining to a temple," from fanum "temple, shrine, consecrated place," related to festus "festive" (see feast (n.)). Meaning "zealous person, person affected by enthusiasm" is from 1640s. As an adjective, in English, 1530s, "furious;" meaning "characterized by excessive enthusiasm," especially in religion (of Nonconformists), is from 1640s.
A fanatic is someone who can't change his mind and won't change the subject. [attributed to Winston Churchill]
1650s, "pertaining to a feast," from Latin festivus "festive, joyous, gay," from festum "festival, holiday," noun use of neuter of adjective festus "joyful, merry" (see feast (n.)). The word is unattested in English from 1651 to 1735 (it reappears in a poem by William Somervile, with the sense "fond of feasting, jovial"), and the modern use may be a back-formation from festivity. Meaning "mirthful, joyous" in English is attested by 1774. Related: Festively; festiveness.
When the Day crown'd with rural, chaste Delight
Resigns obsequious to the festive Night;
The festive Night awakes th' harmonious Lay,
And in sweet Verse recounts the Triumphs of the Day.
[Somervile, "The Chace"]
Earlier adjectives in English based on the Latin word were festival "pertaining to a church feast" (late 14c.); festful "joyous" (early 15c.), festial "pertaining to a church feast" (early 15c.), festli "fond of festivity" (late 14c.).
It forms all or part of: apotheosis; atheism; atheous; Dorothy; enthusiasm; fair (n.) "a stated market in a town or city;" fanatic; ferial; feast; fedora; -fest; festal; festival; festive; festoon; Festus; fete; fiesta; henotheism; monotheism; pantheism; pantheon; polytheism; profane; profanity; Thea; -theism; theist; theo-; theocracy; theodicy; Theodore; Theodosia; theogony; theology; theophany; Theophilus; theosophy; theurgy; tiffany; Timothy.
It is the hypothetical source of/evidence for its existence is provided by: Greek theos "god;" Latin feriae "holidays," festus "festive," fanum "temple."
1660s, "pertaining to or of the nature of a feast," from Late Latin convivialis "pertaining to a feast," from Latin convivium "a feast," from convivere "to carouse together, live together," from assimilated form of com "with, together" (see con-) + vivere "to live" (from PIE root *gwei- "to live"). Meaning "sociable" is from 18c. Related: Convivially; conviviality.
"to extort entertainment from, to levy extractions upon," 1630s, apparently a phonetic spelling of Irish coisir "feast, entertainment." Related: Coshering (1570s).