Many people called him that because he never really showed what he really thought. He was contrary to his name a very talkative person.

R. Vermeir 'Een inleiding tot de geschiedenis van de vroegmoderne tijd'

likely revisionist after looking further

It's apparently a misreading of his Dutch nickname "schluwe" into English as silent, it actually meant prudent or cunning but the English translation to silent stuck ref

It seems to have been attached to William by Antoine Perrenot, know as Cardinal de Granvelle, the first mention of the nicknameappear in his correspondence with Phillp II.

There is a lot of information about the nickname in French, William is known as Guillaume le Taciturne. The correspondence doesn't translate well but the sources are available, pretty cool for 500 years ago. It seems the conflicting stories are based on the viewpoint of being for or against William of Orange.

CORRESPONDANCE DE PHILIPPE II LES AFFAIRES DES PAYS-BAS; 1559-1567

https://archive.org/stream/correspondancede03phil/correspondancede03phil_djvu.txt

559] Diplomatic Efforts. 109

It was from the control he kept over himself that day in the Vincennes forest that the prince gained the name of le Taciturne or the Silent. The epithet was bestowed for a single action, and not for an habitual trait, This story is mentioned in Pontus Payen, i., 8; see also Cor-respondance de Granvelle, vol. vi. , p. 567 et seq.

Memoir es de Pontus Payen, publiees par Alex. Henne. Brussels : 1861, vol. i., p. 9.

http://archive.org/stream/cu31924026955140/cu31924026955140_djvu.txt

408 THE RISE OF THE DUTCH REPUBLIC [1567

In conversation with those abont him he frequently expressed regret that the Prince of Orange had been too crafty to be caught in the same net in which his more simple companions were so inextricably entangled. Indeed, on the first arrival of the news that men of high rank had been arrested in Brussels, the cardinal eagerly inquired if the Taciturn had been taken, for by that term he always characterized the prince. Receiv- ing a negative reply, he expressed extreme disappointment, adding that if Orange had escaped they had taken nobody, and that his capture would have been more valuable than' that of every man in the Netherlands.2

1 Correspondance de Philippe II., i. 674. 2 Hoofd, iv. 151. Strada, i. :iOO. Meteren, 50. 3 Correspondance de Philippe II., i. 523. http://archive.org/stream/completeworksmo02motl/completeworksmo02motl_djvu.txt