(your) lips are sealed

you will not talk about something.

He acts like he wants you to ask what happened, and then if you do ask, he tells you his lips are sealed.

• in the grand scheme of things
• in the great scheme of things
• in the grip of
• in the grip of something
• in the groove
• in the hands of
• in the hands of someone
• in the heat of
• in the heat of something
• in the heat of the moment
• in the hole
• in the hot seat
• in the house
• in the interim
• in their infinite wisdom
• in their wisdom
• in the know
• in the lap of luxury
• in the light of
• in the light of something
• in the line of duty
• in the line of fire
• in the long haul
• in the long run
• in the long term