The second syllable (also in Old Frisian alond, Middle Dutch eiland) was added later to distinguish it from homonyms, especially Old English ea "water" (see ea). As an adjective from 1620s.
Spelling modified 16c. by association with similar but unrelated isle. Similar formation in Old Frisian eiland, Middle Dutch eyland, German Eiland, Danish öland, etc. In place names, Old English ieg is often used of "slightly raised dry ground offering settlement sites in areas surrounded by marsh or subject to flooding" [Cambridge Dictionary of English Place-Names].
Island universe "solar system" (1846) translates German Weltinsel (von Humboldt, 1845). An Old English cognate was ealand "river-land, watered place, meadow by a river." Related: Islander.
1580s as a type of knitted cloth; 1842 as a breed of cattle; both from Jersey, one of the Channel Islands. Its name is said to be a corruption of Latin Caesarea, the Roman name for the island (or another near it), influenced by Old English ey "island" (see island); but it is perhaps rather a Viking name (perhaps meaning "Geirr's island").
The meaning "woolen knitted close-fitting tunic," especially one worn during sporting events, is from 1845. In American English, short for New Jersey from 1758. Related: Jerseyman.
island in the western Pacific, by 1896, of unknown origin; formerly known as Pleasant Island (1798). Related: Nauruan (1918).