1580s, "under the influence of the planet Jupiter," from French jovial (16c.), from Italian joviale, literally "pertaining to Jupiter," and directly from Late Latin Iovialis "of Jupiter," from Latin Iovius (used as genitive of Iuppiter) "of or pertaining to Jupiter," Roman god of the sky (see Jove). The meaning "good-humored, merry," is from the astrological belief that those born under the sign of the planet Jupiter are of such dispositions. Related: Jovially.
c. 1600, "to be jovial and boisterous," also "to talk bombastically," from Dutch randten (earlier ranten) "talk foolishly, rave," of unknown origin (compare German rantzen "to frolic, spring about," dialectal rant "noise, uproar"). Related: Ranted; ranting. Ranters as the name of an antinomian sect which arose in England c. 1645 is attested from 1651; applied 1823 to Primitive Methodists. A 1700 slang dictionary has rantipole "a rude wild Boy or Girl" (also as a verb and adjective); to ride rantipole meant "The woman uppermost in the amorous congress" [Grose].
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.).
pirate flag, attested under that name by 1724, of unknown origin; jolly here has its otherwise obsolete sense "high-hearted, gallant." Also see Roger, the sense of which here is, again, uncertain. A glossary of Banffshire words compiled by the Rev. Walter Gregor and published in 1866 gives a definition of Rodger as "anything of its kind large and ugly," also "Any animal big and ugly," also "A big person of rude manners." It also has a verb rodger "to beat with violence." Perhaps there is a connection.
Their Black-Flag, under which they had committed abundance of Pyracies, and Murders was affix'd to one Corner of the Gallows ; It had in it the Portraiture of Death, with an Hour Glass in one Hand, and a Dart in the other striking into a Heart, and Three Drops of Blood delineated as falling from it : This Flag they call'd Old Roger, and used to say, They would live and die under it. [from a description of the execution of 26 pirates in Rhode Island July 26, 1723, in Historical and Political Monthly Mercury, November 1723]
For the use of jolly, compare Jolly robin "handsome or charming man, gaily dressed man, carefree dandy" (late 14c.) also French roger-bontemps "jovial, carefree man" (15c.).