1670s, "of or pertaining to the French," from Latin Gallicus "pertaining to Gaul or the Gauls," from Latin Gallia "Gaul" and Gallus "a Gaul" from a native Celtic name (see Gaelic), though some connect the word with prehistoric West Germanic *walkhoz "foreigners" (see Welsh). Originally used in English rhetorically or mockingly for "French." The cock as a symbol of France is based on the pun of Gallus "a Gaul" and Latin gallus "cock" (see gallinaceous). Earlier was Gallican (1590s).
It means not simply 'French,' but 'characteristically', 'delightfully', 'distressingly', or 'amusingly' 'French' ... not 'of France', but 'of the typical Frenchman'. [Fowler]
As "of or pertaining to the ancient Gauls" from 1796.
Trojan youth taken by Zeus as his cup-bearer (and lover), from Greek Ganymedes, perhaps a non-Greek name, or from ganymai "I rejoice, am glad" (related to ganos "brightness; sheen; gladness, joy; pride") + medea (plural) "counsels, plans, cunning" (see Medea); taken in Greek folk-etymology to mean "delighting in genitals."
Used figuratively of serving-boys (c. 1600) and catamites (1590s). Associated with Aquarius in the zodiac. As the name of one of the four large satellites of Jupiter, proposed in Latin 1610s, but not widely used before 1847.