Definition of "lethe"

So in the Lethe of thy angry ſoule, Thou drowne the ſad remembrance of thoſe wrongs, Which thou ſuppoſest I haue done to thee.

c. 1593 (date written), William Shakespeare, “The Tragedy of Richard the Third: […]”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act IV, scene iv], page 197

Till that the conquering Wine hath ſteep't our ſenſe, In ſoft and delicate Lethe.

c. 1606–1607, William Shakespeare, “The Tragedie of Anthonie and Cleopatra”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act II, scene vii], page 351

What does it mean to say that the stream of silence originates in lethe? It means, above all, that the stream has its source (Quelle) in that which has not yet been said and which must remain unsaid: the "unsaid."

1980, Joseph J. Kockelmans, On Heidegger and Language, Northwestern University Press, page 241

Pardon me Iulius, here was't thou bay'd braue Hart, Heere did'ſt thou fall, and heere thy Hunters ſtand Sign'd in thy Spoyle, and Crimſon'd in thy Lethee.

1599 (first performance), William Shakespeare, “The Tragedie of Iulius Cæsar”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act III, scene i], page 120, line 219