This is easily one of the greatest lines in King Lear (most of the play SUCKS). The context of this line is Albany is complaining about the treatment of Lear by his daughter (and Albany's wife) Goneril. At which the Evil daughter Goneril tells him to "Marry, your manhood" which is basically the Shakespearean equivalent to "Fuck off".

Aside:
Some other lines that I think would have fit well:

• Go know yourself
• Go play with thy coxcomb
• Dot thy self