What was Big Willy Shakes going for?
So we get the literal meaning of all this. Be true to yourself. Don't borrow money to buy things you cannot afford. Quoted out of context, all of sounds like good advice. "Neither a borrower or a lender be"? Sure, we can get behind that. "To thine own self be true"? Yep, that also sounds like pretty solid advice.
Given that Polonius is the one telling us (or Laertes) these little tidbits of wisdom, are we supposed to take it seriously? Is Shakespeare actually making fun of this cheery, cliché, How to Win Friends and Influence People type of early self-help? (You just know Polonius would have written a self-help book filled with this stuff.)
As comical and ridiculous as Polonius is, his elaborate attempts to keep tabs on Laertes and Ophelia remind us that fathers can't always be trusted to care for their children in the proper way. This is especially true for dear old Polonius who pays Reynaldo to spread rumors about Laertes so that Laertes will confide in Reynaldo, and then Reynaldo can report back to Polonius (2.1.4). Way to keep it classy, Mr. P.
And Polonius doesn't stop there. He's also all-too willing to use his daughter to get in good with the king—with disastrous consequences. His manipulative tactics leave Ophelia open to Hamlet's abuse and are probably partly to blame for Ophelia's tragic end.
So we ask again: are we supposed to take his advice seriously? Well, we'll leave that one to you, Shmoopers. We don't have all the answers. But we do know that Shakespeare is totally making fun of Polonius by giving him tone-deaf advice, a nasty habit of spying on pretty much everyone, and just about the worst sucking up to the king we've ever seen.
Still, maybe there is something to his advice. After all, if he would have listened to his own lines here, maybe the play wouldn't have ended in tragedy for him and his family. The irony is that he's not true to himself. And it gets him killed. And his son and daughter killed, too. Whoops.