From Middle English slynesse. By surface analysis, sly +‎ -ness.

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• Rhymes: -aɪnəs

slyness (countable and uncountable, plural slynesses)

1. (uncountable) The state or quality of being sly.

   With his natural slyness, he was able to talk his way out of trouble.

2. (countable) The result or product of being sly.

• craftiness

• selsyns