The phrase "letted know" does not have an established antonym in the English language. However, there are several alternative phrases that convey a similar meaning, such as "withheld information," "kept in the dark," or "failed to inform." Each of these antonyms implies a situation where a person intentionally chooses not to share knowledge or communicate with someone else. In contrast, letting someone know indicates openness and transparency in sharing information. Depending on the context, an antonym for "letted know" might suggest secrecy, deception, or incompetence. It is always important to consider the specific circumstances and implications of language use before selecting an antonym.