Ne pas y aller de main morte

Literally: Not to go at it with a dead hand

Example: “Le critique n'y est pas allé de main morte avec ce film.” — The critic didn't pull any punches with that movie.

English equivalent: To pull one's punches

Origin: The image is literal and rests on a comparison. A hand severed from its arm is a main morte, a dead hand: entirely inert, incapable of so much as swatting a mosquito. Set against that, the hand on the end of a living and none-too-gentle arm is emphatically alive, and can do real damage. That contrast was enough for French to say, from the 17th century, that a man who struck like that was not going about it with a dead hand at all. The second sense — acting brutally, with no restraint, in any context — is just the metaphor extended off the fist.