The adjective Atlantic originally referred to Mount Atlas*, on which the heavens were fabled to rest. It was hence applied to the sea near the western shore of Africa, and afterwards extended to the whole ocean lying between Europe and Africa on the east and America on the west.
* The Atlas Mountains are a range of mountains in North Africa extending from Morocco to Tunisia in a series of chains. The ancient writers considered Atlas not as a chain of mountains, but as an isolated mountain, surrounded by sands. For example, in Naturalis Historia (The Natural History – 77), a vast encyclopaedia of the natural and human worlds, the Roman statesman and scholar Pliny the Elder (23-79) wrote of
the nation of the Autololes, through whose country lies the road to Mount Atlas, the most fabulous locality even in Africa. It is from the midst of the sands, according to the story, that this mountain raises its head to the heavens; rugged and craggy on the side which looks toward the shores of the ocean to which it has given its name, while on that which faces the interior of Africa it is shaded by dense groves of trees, and refreshed by flowing streams; fruits of all kinds springing up there spontaneously to such an extent, as to more than satiate every possible desire. Throughout the daytime, no inhabitant is to be seen; all is silent, like that dreadful stillness which reigns in the desert. A religious horror steals imperceptibly over the feelings of those who approach, and they feel themselves smitten with awe at the stupendous aspect of its summit, which reaches beyond the clouds, and well-nigh approaches the very orb of the moon. At night, they say, it gleams with fires innumerable lighted up; it is then the scene of the gambols of the Ægipans [= the Fauns] and the Satyr crew, while it re-echoes with the notes of the flute and the pipe, and the clash of drums and cymbals. All this is what authors of high character have stated, in addition to the labours which Hercules and Perseus there experienced.
translation: John Bostock and Henry T. Riley (1855)
Via Latin Atlanticus, Atlantic is from Greek Ἀτλαντικός (= Atlantikos), from Ἄτλας, Ἀτλαντ- (= Atlas, Atlant-), which, before being applied to the mountain, was the name of a member of the older family of gods, who was supposed to hold up the pillars of the universe, and later of one of the Titans, condemned to bear up the heavens, or, in other forms of the legend, the earth.
Atlas has been used figuratively, for instance by the English pamphleteer, playwright and poet Thomas Nashe (1567-circa 1601) in To the Gentlemen Students of Both Vniuersities (1589) about the English writer George Peele (1556-96):
I dare commend him to all that know him as the chiefe supporter of pleasance nowe liuing, the Atlas of Poetrie and primus verborum Artifex, whose first encrease, the Arraignement of Paris, might plead to your opinions his pregnant dexteritie of wit and manifold varietie of inuention, wherein (me iudice) hee goeth a step beyond all that write.
The word atlas in the sense of a collection of maps in a volume is said to be derived from a representation of Atlas supporting the heavens placed as a frontispiece to early works of this kind, and to have been first used by the Flemish geographer and cartographer Gerardus Mercator (1512-94).