Note to Hamlet, 3.2.281-284: "For thou dost know, O Damon dear, / This realm dismantled was / Of Jove himself; and now reigns here / A very, very—pajock"

In Greek mythology Damon was famous for his loyalty and trust in friendship, so perhaps Hamlet is showing his appreciation of Horatio's friendship by calling him Damon.

"This realm dismantled was / Of Jove himself" means "this kingdom was stripped of a king who equaled Jove himself," and I believe that in Hamlet's mind the king of Denmark who equaled Jove himself was King Hamlet, Hamlet's father. And the "pajock" is the current king.

Kenneth Branagh as Hamlet; Nicholas Farrell as Horatio
--1996 film--