fastness

From Middle English fastnesse, festnesse (“firmness; certainty; stronghold; firmament”), from Old English fæstnes, fæstnis (“firmness; stronghold; firmament”), equivalent to fast +‎ -ness.

fastness (countable and uncountable, plural fastnesses)

1. A secure or fortified place; a stronghold, a fortress.
   • 1611, John Speed, The History of Great Britaine under the Conquests of the Romans, Saxons, Danes and Normans, London, Book 9, Chapter 9, p. 528,

     […] if the Welsh compelled by famine ventred out of their strengthes or fastnesses, in or about Snowdon, the Garrison Souldiers of Gannocke were ready to intercept and kill them […]

   • 1803, John Browne Cutting, “A Succinct History of Jamaica” in Robert Charles Dallas, The History of the Maroons, London: Longman and Rees, Volume 1, p. xxxviii,

     […] the slaves that yet remained in the fastnesses of Jamaica, attached to the Spanish, and hostile to the English settlers, continued to be troublesome, and at times formidable.

   • 1917, Edgar Rice Burroughs, “chapter VII”, in A Princess of Mars‎, archive.org, published 1917:

     The incubators are built in remote fastnesses, where there is little or no likelihood of their being discovered by other tribes.

   • 1919, W. Somerset Maugham, The Moon and Sixpence, ch. 4

     When she came to know writers it was like adventuring upon a stage which till then she had known only from the other side of the footlights. She saw them dramatically, and really seemed herself to live a larger life because she entertained them and visited them in their fastnesses.

2. The state of being fast.
   1. Firmness, security.
   2. Rapidity, swiftness.
3. The ability of a dye to withstand fading.

• colorfastness
• colour fastness, colourfastness