I didn't mean to tell anyone you were getting married - it just slipped out.

slipped 是动词 slip 的过去式，slip 本意是"滑落、滑倒"，引申为"无意中发生"。 out 在这里是副词，表示"向外、显露"。 slipped out 这个短语表示"无意中说漏嘴"，就像东西从手中滑落一样不受控制地说出来。