A kenning is a stylistic device that is used by joining two words into a phrase to define an object. This stylistic device can be mostly found in the Old English and Norse poetry.
In Beowulf, Kennings are used extensively. The expressions given in the question are an example of kennings.
The expression "cloud-murk" means an overcast sky with gray clouds. And phrase, "bone-lappings" is used to refer to ligaments in the body that holds the muscles together.
