Literally: Mute and sewn lips
Meaning: Mum's the word; my lips are sealed; keep it under your hat.
Origins: 15th century, used to order somebody to not reveal a secret, otherwise he would have his lips sealed, closed sewn with thread.
Motus supposedly comes from the Latin word “mutus” which mean mute.
Today It would be used to ask somebody for his discretion not to reveal a secret