We can summarize this line as a statement on being careful in the social world. Have tact. Be careful. Be cagey. "Don't put all your cards on the table." At least, try to refrain from exposing yourself in full to the knowledge and the whims of others.

In the context of the play, this is advice that King Lear could have used in the beginning. Tragically, it proves, Cordelia is a foil to the folly of King Lear.

Cordelia can be seen as acting on the advice of humbly presenting herself and she suffers for it. When Lear demands that his daughters compete for the largest share of his kingdom by testifying to their love for him, Cordelia speaks without exaggeration.

"[Lear] receives embellished speeches of endearment from the older two, but Cordelia modestly speaks the truth, angering her father who disinherits her and banishes her forever" (eNotes).

Although Cordelia is banished, she maintains her integrity and finds love. She is the best example of a virtuous person and thus retains her human value, where others (including Lear) struggle and often lose their moral qualities.

Lear, having exposed himself to the caprices of his other two daughters finds no sympathy. He has, in essence, given himself away by proffering his kingdom to Goneril and Regan, going against the advice inherent in the line in question.

"Have more than thou showest, speak less than thou knowest, lend less than thou owest."

Unfortunately for Lear, he does not heed his own wisdom as expressed early in the play. 

"Allow not nature more than nature needs / Man's life is cheap as beasts'"

By giving away the property of his kingdom, he gives away his power and puts it in the hands of those whose natures are, perhaps, unsuited to such wealth and power and so immediately abuse it. He has overestimated the esteem two of his daughters hold for him and has overestimated the quality of their natures, giving them more than they need. 

The result is that Lear is left utterly exposed to the vanity and corruption of Goneril and Regan and ultimately turned away, exposed to the raw powers of nature, alone and mad in the wild. 

This is much like the advice that Polonius gives to his son in Hamlet.

"Have more than thou showest" quite simply means don't go throwing your money around and letting everybody know how much money you have.  In other words be moderate in how you present yourself to the world.

"Speak less than thou knowest" in other words, listen, don't talk.  If you talk to much you might let slip something you don't want people to know or you could "stick your foot in your mouth" so it is better to listen to others and speak only when you have something relavent to say.

"Lend less than thou owest" is almost like "neither a borrower nor a lender be" in the advice.  To lend out money, especially to a friend, is really tricky, particularly in trying to get get your money back sometimes so it is better not to lend your money out.

In all the advice is to keep a low profile and not call attention to yourself through your actions or speech.  Live in moderation.

What is the significance of the quote in King Lear: "Have more than thou showest..."?

The quote "Have more than thou showest, speak less than thou knowest, lend less than thou owest" suggests that it is best to be judicious or sensible when it comes to dealing with others in life relationships. We will look at the three parts of this quote individually as relates to life in general, which is what King Lear is, an examination of a man’s life:

1. Have more than thou showest,

Sometimes it is wise not to reveal to others all that you have. You have to know who you are dealing with and what their motives are. In essence, you are protecting yourself and your family when you think this way. Everyone does not have to know all of your business and what you own.

Also, when we show all that we have, it is sometimes looked upon as being “showy” or flaunting our good fortune and abundant assets. This can invite the jealousy of others.

2. speak less than thou knowest,

It doesn’t have to be physical things either, as mentioned above. In speech, we can flaunt our great knowledge of and expertise in a subject. Sometimes it is better to withhold a bit of what we know so others are not intimidated by this. If they seek to draw more information out of us on a subject because of personal interest, that is fine. It’s just that sometimes it’s better to let them make the first move in this so we don’t appear to be exalting the self.

In addition, when we speak loosely, revealing all, we may lose an advantage that we may have in a touchy situation, or a business situation. This is especially true in negotiations between parties, whether business deals or labour negotiations. Sometimes it is wise to keep some of our thoughts close to the vest until we learn more of the real motives of others, or until we can gain more information before laying all of our cards on the table. If we say too much too soon, others may take advantage of the situation to our detriment.

3. lend less than thou owest.

Here, it is a financial principle being spoken of. It is saying to take money in in greater quantity, as opposed to giving significant sums out. This is open to debate, but it is part of the King Lear story here. Not all would subscribe to this principle in contemporary society.

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In King Lear, this quote is a warning to be sneaky, not showy, to get what you want.

It is not coincidence that the Fool conveys this warning to King Lear in Act I, scene 4.  Fool is a bit of an ironic title.  The battle of the tongue Lear and the Fool engage in relates to the deeper theme of the story, that greed can destroy you.

Let me translate this for you:

Don’t show everyone how much money you have.

Don’t tell everyone everything you know.

Don’t lend money, borrow it!

 Things are not going well with Lear at this point in the story.  He is out of control, and is losing his grip on his kingdom.  Lear's actions are contrary to what he should be doing, quietly calculating how to regain and keep control over his kingdom and family.