ne pas y aller de main morte

To act very forcefully, without holding back

A limp, lifeless hand can't land a real blow, so denying you have one implies you're hitting, or criticising, with real force.

Le critique n'y est pas allé de main morte avec ce restaurant.

The critic didn't hold back at all when reviewing that restaurant.

Expressio traces it to the 17th century, when a properly forceful hand, unlike a lifeless one, was said to strike hard, so denying you had a dead hand meant you were laying it on thick.