General:
* Keen: Having or showing eagerness or enthusiasm.
* Kind: Having or showing a gentle nature and a desire to help others.
* Known: Recognized or familiar.
* Knotty: Difficult to solve or understand.
More specific:
* Kinetic: Relating to motion or movement.
* Kinky: Relating to unusual or unconventional sexual practices.
* Kitschy: Relating to art, design, or culture that is considered to be gaudy or in bad taste.
* Kooky: Eccentric or strange.
* Kosher: Conforming to Jewish dietary laws.
Less common:
* Kraut: (derogatory) Relating to or characteristic of Germans.
* Kurdish: Relating to the Kurdish people or their language.
* Kyphotic: Relating to or having kyphosis (a condition where the spine curves abnormally forward).
Let me know if you'd like more definitions!