It is a kind of quatrain, and probably first attested in fourteenth-century rímur such as Ólafs ríma Haraldssonar.
[2] An example of the form is this verse by Jónas Hallgrímsson, with a translation into the same metre by Dick Ringler.
Hóla bítur hörkubál, hrafnar éta gorið, tittlingarnir týna sál, tarna' er ljóta vorið!
Hillsides raked by raging frost, ravens eating offal, buntings giving up the ghost — God, this spring is awful!
There are many variations on ferskeytt, whose common principle is that they are quatrains with some kind of alternate rhyme.