look like th'innocent flower, but be the serpent under't

To remember this quote use the following mnemonic.

Looks like an innocent flower

But bee

Don't be silly it's a serpent under it

"Look like th'innocent flower, but be the serpent under't." Act 1 scene 5 line 64. Or slightly rewritten "Look like the innocent flower, but be the serpent under it." Lady Macbeth urges Macbeth to conceal his treacherous intentions from King Duncan. This advice comes as the couple first begin plotting Duncan's murder. 

Lady Macbeth's words in this quotation reveal her deeply duplicitous nature. 

The reference to the "serpent" carries religious undertones, alluding to the biblical snake (a symbol of the devil) that tempts Eve in the Garden of Eden. Like the serpent, Lady Macbeth is a woman tempted by evil who in turn tempts a man (Macbeth). In the bible this temptation leads to the fall of man; in Macbeth, it results in the downfall of both Lady Macbeth and her husband. 

Through Lady Macbeth's deception, Shakespeare presents a cautionary tale about the dangers of unchecked ambition and moral corruption.