jazz declaims: It's called a kenning.

so it is; it's a Scottish verbal noun, having several senses; one being: 6. One of the periphrastic expressions used instead of the simple name of a thing, characteristic of Old Teutonic, and esp. Old Norse, poetry.

I particularly enjoyed this citation: 1896 Scott. Rev. Oct. 342 note, The kennings for ‘man’ in Gröndal's Clavis Poetica extend to 33 closely printed columns.