Literally: Farting sideways
Meaning: To be grumpy; to get a fart crosswise
Origins: 12th century, “pet” (fart) comes from the Latin word peditum, from the verb pedere which meant "péter" (to fart). 13th century, it evolves into “quelque chose de peu de valeur (something cheap). The expression really appears in the 20th century, indicating someone grumpy because of having digestives problems.
Synonyms: Souffrir d'une indisposition passagère; Être contrarié, de mauvaise humeur
Examples: Elle a toujours un pet de travers/ she has always something wrong with her