"Rat on" is a colloquial expression that denotes telling on or betraying someone. There are a myriad of synonyms that can be used in place of "rat on", depending on the context it is used in. Some of these synonyms include snitch, tattle, inform, expose, reveal, squeal, divulge, disclose, spill, and blab. Each of these words has a slightly different connotation and usage, but they all share the underlying idea of revealing information that someone else would prefer to keep secret. Choosing the right synonym can help to convey a particular tone or level of severity in the situation being described.