Well, the OED reckons the Netherlands, the relevant entry being:

b.A.2.b pl. The Low Countries.

   c 1685 in Roxburghe Ballads (Ballad Soc.) (1887) VI. ii. 421 And it is called the Sweet Trinity, And was taken by the false Gallaly, sailing in the Low-lands.    1923 G. B. Harrison Shakespeare's Fellows iii. 100 Between his service in the Lowlands and the success of Every Man in his Humour, 1598, he had tried acting.    1961 T. Henrot Belgium 28 Some fifteen Spanish grandees were named successively governors of the Spanish Low Lands.

Of course, whether you invariably believe every word in the OED is another matter...