Montrer de quel bois on se chauffe

Literally: To show what wood one heats oneself with

Example: “Encore une remarque de ce genre et je vais lui montrer de quel bois je me chauffe.” — One more remark like that and I'll show him what I'm made of.

English equivalent: Show someone what one is made of

Origin: When the phrase appeared in the 16th century, wood was the only way to heat a French home, and not all wood burns alike: some logs flare brightly and give little warmth, while oak or hornbeam burn hot and long. Knowing what wood a household burned meant knowing its means and its nature, so "savoir de quel bois quelqu'un se chauffe" first simply meant to know someone's character. From the 17th century the phrase hardened into a warning — "je vais te montrer de quel bois je me chauffe" — perhaps helped along by the log that doubles as a club and by the heat of anger in "se chauffer." It is most often heard in the first person, as the threat itself.