Literally: press on the mushroom.
Meaning: slang for accelerate. To put the pedal to the metal; to put one’s foot down; to step on the gas
Origins: early 20th century, originally, the accelerator in cars was like a metal poll with a ball at the end which looked very much like a mushroom. The expression became popular, it is still used today, even the accelerator pedal is way different.
Synonyms: mettre les gaz; mettre le pied au plancher.