When someone "lets on," they reveal or disclose something, often unintentionally. Other phrases that can be used as synonyms for "let on" include giving away, divulging, leaking, spilling the beans, or betraying a secret. In some cases, "letting on" can also refer to pretending or acting as if, which can be replaced with synonyms such as feigning, simulating, masquerading, or putting on a show. However, it's important to note that the specific meaning of "letting on" can vary depending on the context, so it's always a good idea to consider the full situation before selecting a synonym.