Real Dictionary

swear
If someone is sworn to secrecy or is sworn to silence, they promise another person that they will not reveal a secret.
verb: If someone swears, they use language that is considered to be rude or offensive, usually because they are angry.
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.
verb: If you swear to do something, you promise in a serious way that you will do it.

swear in American English
to make a solemn declaration with an appeal to God or to something held sacred for confirmation
intransitive verb: to make a solemn declaration or affirmation by some sacred being or object, as a deity or the Bible
transitive verb: to declare, affirm, attest, etc., by swearing by a deity, some sacred object, etc.

swear in British English
verb: to declare or affirm (a statement) as true, esp by invoking a deity, etc, as witness