Of, relating to, or characteristic of the Danish island of Zealand, its people, or its dialect. Also as noun: the dialect of Danish spoken in Zealand.
Late 17th century; earliest use found in Philosophical Transactions of the Royal Society of London. From Zealand (also Sealand, (now rare) Zeeland) + -ish, after Danish sjaellandsk (Old Danish siaelandsk, siaellandsk, etc.).