Your slip is showing

Literal; A slip is a woman's undergarment worn usually beneath a dress. Sometimes it is very close to the same length. If the dress 'rides up' such as when sitting someone may say your/her slip is showing.
Figuratively it means that 'you' made a 'slip of the tongue' and spoke of something that shows your true intent or beliefs that may not be the most appropriate at the time. When it is literally used it pertains to women. When it is figuratively used it can be used to address both a woman or a man. Take care that it is not regarded as an insult to masculinity.

Φαίνεται το εσώρουχό σου, για κάτσε καλύτερα!

Η αλεπού κρυβότανε, μα η ουρά φαινότανε

Θέλει η π**** να κρυφτεί, μα η χαρά δεν την αφήνει.