[Ko-kytus ]. One of the five rivers of hell. The word means the “river of lamentation.” The unburied were doomed to wander about its banks for 100 years. (Greek, koku'o, to weep.)
Cocytus, named of lamentation loud Heard on the rueful stream.
Milton: Paradise Lost, ii. 579.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Cocytus from Infoplease: