Explain the quote, 'Look like th'innocent flower, / But be the serpent under't'
She is advising Macbeth on how to go about killing King Duncan. She encourages him to appear innocent and play the welcoming host to the king so that no one will suspect his true intention: murder.
Secondly, to really hammer this point home, Lady Macbeth talks about acting like a flower but really behaving like the serpent/snake underneath it, and this is where things get really interesting. When we think about a flower - about the connotation - we think of some nice, something pleasant to look at. This reinforces Lady Macbeth's point about Macbeth treating his guests warmly and with kindness.
However, when we think of a serpent or a snake, something far more sinister comes to mind. Think about the serpent in the Bible, for example, the snake which tempted Eve to eat the apple in the Garden of Eden. So this section of the quote is actually an allusion to that famous Biblical story, reinforcing the idea of Macbeth's sinister intentions.