Rosencrantz and Guildenstern don’t have much helpful information to share with the royal couple, so King Claudius and Polonius move forward with their plan to spy on Hamlet and Ophelia. Right before Hamlet enters, Polonius says that people who are sinners often use "devotion’s visage and pious action" to "sugar o’er the devil himself." What does Polonius mean?

Polonius meant to imply that human nature tend to act good or pious in front of others by using a religious facade so as to hide our real sinful deeds hidden from being seen.

Explanation:

When Polonius and Claudius decide to spy on Hamlet in conversation with Ophelia in Act III scene i, he asks his daughter to act natural and read from a prayer book so that she may appear pious in front of Hamlet. This act is also accepted by him that with "pious action we do sugar o'er  The devil himself".

By that phrase, he meant to imply that people try to hide their sinful deeds by acting pious/ religious/ righteous in front of everyone. By using "devotion’s visage" as a cover up for the bad actions, we all tend to hide our real motives and actions, "suger(ing) the devil himself", meaning pleasing the devil.

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