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 是副词，表示"向外、出来"。 slipped out 这个短语表示"无意中说漏嘴"，就像东西不小心从手中滑落一样，话语也不经意间脱口而出。