In Macbeth by William Shakespeare, Lady Macbeth advises her husband to "look like the innocent flower, but be the serpent under it." By this, she means that he should appear to be innocent to belie his devious and murderous plans. Yet, despite assuming an innocent appearance, he must remain as murderous or venomous as a serpent. By telling Macbeth to “look like the innocent flower,” she wants him to seem unthreatening and harmless to put his intended victim at ease so that his plot will come as a surprise and others will not suspect him. After all, who would be afraid of a flower?
However, under that deceptive cloak of innocence, he should be ready to strike Duncan as “the serpent under it [the flower]” would. This line is similar to the concept that "looks can be deceptive" or "don’t judge a book by its cover." In fact, Lady Macbeth even says to her husband in that same scene, as she goads him to commit murder in order to attain the throne,
Your face betrays strange feelings, my lord, and people will be able to read it like a book. In order to deceive them, you must appear the way they expect you to look.
Thus, essentially what she means by “look like the innocent flower, but be the serpent under it” is that Macbeth must deceive people—most importantly Duncan—into believing that he is innocent of murderous thoughts. Lady Macbeth is ambitious. She aspires to share the throne once her husband has killed Duncan and so she coaches him. She continues with:
Greet the king with a welcoming expression in your eyes, your hands, and your words. You should look like an innocent flower, but be like the snake that hides underneath the flower.
Lady Macbeth is chilling in her advice to her husband. Having thus instructed him, the stage is set, so to speak, for Macbeth to kill the king and afterwards appear to have had nothing to do with his assassination.