Canker-worm

A destructive grub or caterpillar. Buttercup is speaking figuratively and hinting at her dark secret. Her words remind us of Shakespeare’s lines in Twelfth Night: “She never told her love, but let concealment, like a worm i’ th’ bud, feed on her damask cheek.”

Context: 
there may lurk a canker-worm
Opera: 
Act: 
Act I