"Look like the innocent flower, but be the serpent under't."

Definition: Please see notes below.

Notes:

...look like the innocent flower,
But be the serpent under't. (1.5.75)

Some editors believe these lines derive from Virgil's Eclogues 3.93, "You, picking flowers and strawberries that grow
So near the ground, fly hence, boys, get you gone!
There's a cold adder lurking in the grass."

These editors have overlooked the contemporary event to which Shakespeare here refers. In The Royal Play of Macbeth (the most fascinating and convincing book I have ever read on Shakespeare's authorial intention) Henry Neill Paul explains that these lines are an allusion to arguably the most significant moment in Shakespeare's time, the discovery of the Gunpowder Plot. To commemorate the discovery of the heinous scheme, King James had a medal created picturing a serpent hiding amongst flowers. Every person watching Shakespeare's drama in Jacobean England would have understood immediately the context and weight of such an allusion, King James most of all. For more on Shakespeare and allusions to the Gunpowder Plot in Macbeth, please see my article here. For more on how and why Shakespeare shaped Macbeth to please James I, see my article here.