From Greek Mythology:
* Achilles' heel: A vulnerable spot. Achilles was a hero of the Trojan War who was invulnerable except for his heel, which was dipped in the River Styx by his mother.
* Pandora's Box: A source of great evils. Pandora was a woman who opened a box that released all the evils of the world into it.
* The Gordian Knot: A complex problem that seems impossible to solve. Gordius, a king of Phrygia, tied an extremely intricate knot and declared that whoever untied it would rule Asia. Alexander the Great solved the problem by simply cutting the knot with his sword.
* To have a Midas Touch: To be able to turn everything you touch into gold. Midas, a king of Phrygia, was granted the power to turn everything he touched into gold, but he soon realized the curse of his wish.
* Herculean Task: A very difficult or demanding task. Hercules was a legendary hero who was known for his incredible strength and was tasked with completing twelve impossible labors.
* Siren Song: Something that is very attractive but dangerous. Sirens were mythical creatures who lured sailors to their deaths with their beautiful singing.
* Sisyphus's Labor: A task that is seemingly endless and futile. Sisyphus was a king who was condemned in the underworld to roll a boulder uphill for eternity, only to have it roll back down again each time he reached the top.
From Roman Mythology:
* Janus-faced: Having two opposing aspects. Janus was the Roman god of beginnings, endings, transitions, doorways, passages, time, duality, and gates. He was often depicted with two faces, one looking forward and the other looking backward.
* Romancing the Stone: To be on a romantic adventure or journey. Roman gods were said to be able to shape-shift and transform themselves into animals, especially wolves.
* Crossing the Rubicon: To make an irreversible decision. The Rubicon was a river that marked the boundary between Gaul and Italy. Julius Caesar, a Roman general, famously crossed the Rubicon with his legions, defying the orders of the Senate and effectively starting a civil war.
From Norse Mythology:
* To have a wolf in your stomach: To be extremely hungry. Fenrir, a monstrous wolf in Norse mythology, was prophesied to kill the god Odin during Ragnarök, the end of the world.
* To be caught in a bind: To be in a difficult situation. The Norse god Thor was once caught in a bind (literally) by the giant Loki when Loki tricked him into wearing a magic belt that made him unable to move.
* To be in Valhalla: To be in a state of glory and bliss. Valhalla is the hall of slain heroes in Norse mythology, where they feast and fight in preparation for Ragnarök.
From Other Mythologies:
* The Emperor's New Clothes: A metaphor for people who pretend to have something they do not, in order to impress others. This story comes from a European folktale.
* A monkey wrench thrown into the works: A sudden unexpected problem. This expression originates from the era of industrial machinery where throwing a wrench into the works of a machine would bring it to an abrupt stop.
This is just a small sampling of phrases and idiomatic expressions with mythological origins. There are many more out there, and the meaning of some may be obscured over time. However, understanding the origins of these expressions can help us to better understand their meaning and appreciate the richness of language.