"of or pertaining to Portuguese navigator Fernão de Magalhães (c. 1470-1521), the first European to round the tip of South America, whose surname was Englished as Magellan.
The Magellanic Clouds, the two cloud-like patches of stars in the southern heavens, are attested under that name by 1680s. Magellan described them c. 1520, hence the name in Europe; but at least the larger of the two had been mentioned in 1515 by Peter Martyr d'Anghiera, chronicler of explorations in Central and South America.
In English they were earlier the Cape Clouds, because they became prominent as sailors rounding Africa neared the Cape of Good Hope; "but after Magellan became noted and fully described them they took and have retained his name." [Allen]
Coompasinge abowte the poynt thereof, they myght see throughowte al the heaven about the same, certeyne shynynge whyte cloudes here and there amonge the starres, like unto theym whiche are scene in the tracte of heaven cauled Lactea via, that is the mylke whyte waye. [Richard Eden, translation of "Decades of the New World," 1555]