“Even a worm will turn” is an expression used to mean that an unfavorable situation can be reversed, that even the meekest or most docile of creatures will retaliate or get revenge if pushed too far.
It’s a metaphor meaning that a person will fight back if pushed too far. I don’t think worms fight back, but remember that “worm” is another name for “dragon.” In fairy tales, the flying dragon spewing fire would ravage fields and villages. To be in the dragon’s path resulted in inescapable destruction. Everyone would hope for it to change direction or to “turn.”
The phrase was first recorded in a 1546 collection of proverbs by John Heywood, in the form “Treade a worme on the tayle, and it must turne agayne.” The meaning was that even the most humble creature tries to counteract rough treatment.
Shakespeare used the thought in Henry VI, Part 3, where Lord Clifford urges the king against ‘lenity and harmful pity, saying:
To whom do lions cast their gentle looks?
Not to the beast that would usurp their den.
The smallest worm will turn being trodden on,
And doves will peck in safeguard of their brood.'”
I’m sure that the phrase was in use long before 1546, because it describes an ancient and universal truth: that a bullied human, or animal, may well turn savage. In my writing, I mentally apply the phrase to describe my short stories about some downtrodden person suddenly lashing out and getting revenge.
The Worm Turns is the title of a Mickey Mouse cartoon that was released in 1937, and it’s also been used as a song title.