be sworn to something

4. VERB : usu passive

If someone is sworn to secrecy or is sworn to silence, they promise another person that they will not reveal a secret.

• She was bursting to announce the news but was sworn to secrecy.

swear someone to something: to cause someone to take an oath pledging something, such as silence or secrecy, about something.

• I swore Larry to secrecy, but he told anyway.
• We were sworn to silence about the new product.

cf. 3. VERB

If you say that you swear that something is true or that you can swear to it, you are saying very firmly that it is true.

• I swear I've told you all I know.
• I swear on all I hold dear that I had nothing to do with this.
• Behind them was a confusion of noise, perhaps even a shot, but he couldn't swear to it.

cf. swear something to someone: to pledge or promise something to someone.

• I had to swear my allegiance to the general before I could become one of his bodyguards.
• We swore our loyalty to our country.

.... Cobuild & http://idioms.thefreedictionary.com/swear