Could someone help me understand something Hamlet says to Polonius?

For the satirical rogue says here that old men
have gray beards, that their faces are wrinkled, their eyes
purging thick amber and plum-tree gum, and that they
190 have a plentiful lack of wit, together with most weak
hams—all which, sir, though I most powerfully and
potently believe, yet I hold it not honesty to have it thus set
down; for yourself, sir, should be old as I am, if like a crab
you could go backward.

I just really don't understand the last part about going backward? I'm reading the archaic text next to the modern just in case, but the modern is pretty similar to it. And since this precedes such a famous line, I'd really like to know this part. Thanks in advance.

This site discusses various interpretations of this line.

http://bardseyeviewblog.blogspot.com/2006/01/hamlet-8-madness-yet-theres-method.html

For the satirical rogue says here that old men
have gray beards, that their faces are wrinkled, their eyes
purging thick amber and plum-tree gum, and that they
have a plentiful lack of wit, together with most weak
hams—
He is describing what "the satirical rogue" says old men should be like.
all which, sir, though I most powerfully and
potently believe, yet I hold it not honesty to have it thus set
down;
But Hamlet doesn't believe it is true of all old men.
for yourself, sir, should be old as I am, if like a crab
you could go backward.
If Polonius could go backward in time, he would be like Hamlet is.

So ... who is that satirical rogue? And do you think Hamlet is speaking what he really thinks? Or is he making fun of Polonius in a cutting way? Remember that he is pretending to be "mad" in this section of the play.

??

In this passage, Hamlet is speaking to Polonius and responding to the satirical comments made by "the satirical rogue." The "satirical rogue" is a character or figure who is known for making mocking or sarcastic remarks about old men. Hamlet is describing what this satirical figure says about old men: that they have gray beards, wrinkled faces, eyes that ooze thick amber and plum-tree gum, and a lack of wit and weak hams (referring to weak legs or lack of strength).

Hamlet then says that although he most powerfully and potently believes what this satirical rogue claims about old men, he does not think it is honest or fair to generalize and stereotype all old men in this way.

The last part, where Hamlet says, "for yourself, sir, should be old as I am, if like a crab you could go backward," is a sarcastic comment directed at Polonius. Hamlet is essentially saying that if Polonius, who is older than Hamlet, could go back in time and be as young as Hamlet, he would not age gracefully but rather regress or deteriorate, like a crab moving backward.

It's important to consider the context of this passage. At this point in the play, Hamlet is feigning madness as part of his larger plan. Therefore, it is possible that his comments are meant to be cutting and mocking towards Polonius, who he sees as a foolish and meddling old man. Hamlet's words may not necessarily reflect his genuine beliefs about old age, but rather serve as a tool for his performance of madness.

For further analyses and interpretations of this line and its surrounding context, you can refer to the link you provided: http://bardseyeviewblog.blogspot.com/2006/01/hamlet-8-madness-yet-theres-method.html