Demeter (or Ceres in the Roman tradition) is the Greek Goddess of the harvest. Other traditions describe her as ruler of the poppy, a guardian of the underworld, and, more generally, mother Earth. Sometimes she is depicted in flowing robes holding grain. Other times she is vengefully bringing about the first winter. She is perhaps most famously known as the mother of Persephone.
Many of us are familiar with her daughter’s tale. Persephone is abducted by the God of the underworld, Hades, who wants her to be his wife. He forcefully brings her to the underworld. Abductions and rapes were common among the Greek Gods, and this was not depicted negatively in the original story. This act causes her mother Demeter so much grief that she stops all life on the planet from growing, and places the fate of ole humanity in the crossfires. The King of the Gods, Zeus, eventually manages to broker a deal between Hades and Demeter where they split custody of Persephone 50–50. This tale becomes a mythological explanation for the change of seasons. Persephone spends half her time in the underworld (winter), and the other half in the world of the living (spring).
If we look at the tale from Demeter’s perspective, however, we see the story as a mother bending heaven and Earth to protect her child. Even though by the misogynistic standards of Greek lore, Hades is in the right, Demeter’s love for her child propels her to take direct action; something that women of the period would normally be shunned for. Hades kidnaps her daughter, and she holds the entire world of man hostage. She is willing to let all of humanity die to get her daughter back, and it (mostly) works. In the process, she demonstrates how much a patriarchal society believes a mother should be willing to sacrifice to protect her child: she should be ready to give up everything.
In the modern day, this is called the Mama Bear trope, and as we have just demonstrated, its legacy is thousands of years old. The phrase’s contemporary origin comes from the meme that bears are generally passive in the wild, but if you provoke them, they will attack. Likewise, the patriarchal conception of feminity portrays mothers as passive, but if you get between them and their children, they will do everything within their power to destroy you.