What is "ifingr"

In Norse mythology, Ifing ( Old Norse, Ífingr) is the name of a river that separates Asgard, the realm of the gods, from Jotunheim, the land of giants, according to stanza 16 of the poem Vafthrudnismal from the Poetic Edda:

"Ifing the river is called, which divides the earth between the sons of giants and the gods; freely it will flow through all time, ice never forms on the river." — Larrington trans.

John Lindow in Norse Mythology (2001) states in reference to Ifing that a river on which ice will never form is one that runs swiftly and therefore is extremely difficult to ford (thus forming an effective barrier between the worlds of gods and giants).