Casser du sucre sur le dos de quelqu’un can literally be translated by “breaking sugar on someone’s back”. It is used when someone engages in malicious gossip about someone else, in other words criticizes someone. The idioms appeared in the late 19th century and might come from the verb sucrer quelqu’un, which used to mean “mistreating someone”.
Casser du sucre sur le dos de quelqu’un
French with Valerie 