and Someone had better keep still about it.; Better keep quiet about it.; Someone better keep quiet about it.
A particular person ought not to tell about or discuss something. (The someone can stand for any person's name, any pronoun, or even the word someone meaning "you-know-who." If there is no Someone had, the phrase is a mild admonition to keep quiet about something.) Mary: I saw you with Bill last night. Jane: You'd better keep quiet about it.Jane: Tom found out what you're giving Sally for her birthday. Bill: He had better keep quiet about it!