Casser du sucre sur le dos de quelqu'un

Literally: To break sugar on someone's back

Example: “Dès que le patron sort, ses collègues cassent du sucre sur son dos.” — As soon as the boss leaves, his coworkers badmouth him behind his back.

English equivalent: To talk behind someone's back

Origin: First recorded in exactly this form in 1868, though the pieces behind it are older. Sugar was once sold as a solid loaf that had to be broken into usable pieces as needed, which supplies the casser — a repeated, small, deliberate action, not one clean blow. Separately, the 18th-century Dictionnaire de Trévoux records se sucrer de quelqu'un meaning to take someone for a fool, and slang had casser du sucre on its own for spreading gossip or treating someone roughly. The two strands merged, and sur le dos was added to make explicit what the shorter form only implied: that the person being talked about isn't there to hear it, and has to carry the cost of the conversation on their back regardless.