casser du sucre sur le dos de quelqu'un

to gossip maliciously about someone

This expression means to talk badly about someone behind their back or engage in malicious gossip.

Elles passent leur temps à casser du sucre sur le dos de leurs collègues.

They spend their time gossiping about their colleagues.

First recorded in 1868. In the 16th century, sugar came in large solid blocks that had to be broken apart. By the 17th century, 'sucrer' was slang for mistreating someone. The phrase shifted from 'sur la tête' (on the head) to 'le dos' (the back) to capture the idea of talking behind someone's back.