Definition of "disloyal"

Good your Grace, let not any light fancy, or bad Counsel of mine enemies withdraw your Princely favour from me; neither let that stain, that unworthy stain of a disloyall heart towards your good Grace, ever cast so foul a blot on your most dutifull Wife, and the Infant Princesse your daughter […]

1536, Anne Boleyn, letter addressed to Henry VIII from the Tower of London, cited in Edward Herbert, The Life and Raigne of King Henry VIII, London: Thomas Whitaker, 1649, p. 383

[…] Norway himself,With terrible numbers,Assisted by that most disloyal traitorThe thane of Cawdor, began a dismal conflict;

c. 1606 (date written), William Shakespeare, “The Tragedie of Macbeth”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act I, scene ii]

He told his mother he was glad to be back again. He sometimes felt as if it were disloyal to her for him to be so happy with Mrs. Erlich.

1923, Willa Cather, One of Ours‎, Book One, Chapter 15

Embarrassed about leaving him, I asked him to accompany me. It would have been disloyal to let him broil in the heat of Cairo, while I went off to a summer resort.

1998, Tawfiq al-Hakim, “My Donkey and Hypocrisy”, in William Maynard Hutchins, transl., In the Tavern of Life and Other Stories‎, Boulder, Colorado: Lynne Rienner Publishers, page 65