Literal Translation
To believe oneself to have come out of Jupiter’s thigh
Actual Meaning
To think one is the best thing since sliced bread
Etymology
There are no sources citing the exact date of origin of the expression, but the archives of the BnF show that it was in use as early as the 18th century.
To understand this expression, one needs to know a little about Greek mythology. The expression transposed the Greek gods into Latin. The phrase is based on a somewhat far-fetched (and messed up!) story, the kind that the Greeks were known for inventing.
This story is about the birth of Bacchus (Dionysus), god of wine and vines. Jupiter (Zeus), god of gods and supreme ruler of Olympus, was known to have a certain penchant for extramarital affairs. He falls in love with a young woman, Semele, daughter of the king of Thebes, and impregnates her. Juno (Hera), Zeus’s wife, mad with jealousy, decides to pose as Semele’s nurse and persuades her to ask Jupiter to appear in all his splendour. But this vision is unbearable for the eyes of a mere mortal: Semele loses her life. In order to save the child carried by his mistress, Jupiter decides to extract it from its mother’s womb (six months pregnant), cut into his thigh, and place the foetus there so that he can carry the pregnancy to term. The child was thus born from Jupiter’s thigh, proving his glorious and superior origin.