No, it's not an English idiom. There are several similar idioms you can use:

Keep this between us.

Keep this on the qt ("quiet").

Keep this on the dl ("down low")

Keep this hush-hush.

Not everyone will be familiar with these idioms, so to avoid confusion you can use the literal "speak privately".

Update:

Let's talk away from prying eyes.

(thanks to not store bought dirt for the suggestion)

No, it's not an English idiom. There are several similar idioms you can use:

Keep this between us.

Keep this on the qt ("quiet").

Keep this on the dl ("down low")

Keep this hush-hush.

Not everyone will be familiar with these idioms, so to avoid confusion you can use the literal "speak privately".

Update:

Let's talk away from prying eyes.

(thanks to not store bought dirt for the suggestion)

No, it's not an English idiom. There are several similar idioms you can use:

Keep this between us.

Keep this on the qt ("quiet").

Keep this on the dl ("down low")

Keep this hush-hush.

Not everyone will be familiar with these idioms, so to avoid confusion you can use the literal "speak privately".

No, it's not an English idiom. There are several similar idioms you can use:

Keep this between us.

Keep this on the qt ("quiet").

Keep this on the dl ("down low")

Keep this hush-hush.

Not everyone will be familiar with these idioms, so to avoid confusion you can use the literal "speak privately".

Update:

Let's talk away from prying eyes.

(thanks to not store bought dirt for the suggestion)